Have you ever come into possession of a really awesome piece of clothing that you love absolutely everything about except for the color? This happened to me recently, so I decided to overdye it. Here’s a roundup of all the Internet “Research”™ I aggregated in the process of seeing how close I could get to the color I wanted. (Answer: not perfect, but pretty close.)
(“Research” is in quotes to differentiate between fact-checking things presented as unshakable facts, such as chemistry reasoning, vs. applied crafting suggestions which you can take or leave. For the latter, I just read a lot of manufacturer instructions and blogs, with the aim to discern “did this work fine for some people”.)
Types of fiber
You need different types of dye for synthetic fibers vs natural fibers. In addition, different treatments are optimal for natural protein fibers, aka animal hair, vs natural plant fibers, aka cellulose. Your results on blends may vary in efficacy.
- Synthetic fibers: polyester, nylon, lycra/spandex
- Natural protein fibers: silk, wool, cashmere
- Natural plant fibers: cotton, rayon, bamboo, linen, viscose, modal / lyocell / tencel
(Although rayon, viscose, and modal fibers are artificially extruded, they are still natural fibers, made from cellulose.)
- Fabrics described as “satin” or “silk” can often be synthetic imitations, so check the material content tag.
Dyes (adding color)
You can have fun times and delightfully unexpected results with any dye, including natural vegetable dyes, Kool-aid, onions, and tea. However, I will pull out information here under the assumption you are doing small-batch home dyeing while optimizing for color control (dyeing with a specific result in mind), colorfastness (ability to hold up in the wash), and color intensity (which requires some extra steps to attain with natural dyes).
- Fiber-reactive dye is a type of permanent dye that works best on plant-based fibers, and will also work well on protein fibers.
Procion MX is supposedly highly colorfast, and can be used at temperatures that won’t damage fabric (105F / 41C).
- Acid dye works best on protein fibers. (It’s not the caustic variety of acid; the solution is about as acidic as vinegar.) It requires hot temperatures near boiling, so your fabric will shrink if you use it.
Jacquard Acid Dye
- Disperse dye will work on many synthetics, but look up your specific material to see what the recommended options are.
Jacquard iDye, RIT DyeMore
- All-purpose dye is easy to use, but may not give as vibrant/colorfast results.
RIT All-Purpose Dye (works on blends which are less than 35% synthetic)
Dyes are often used with different dye fixatives which promote the dye reaction or binding in various ways. The dye you pick should tell you which particular fixative it recommends. Certain dyes will also recommend adding non-iodized salt to help improve dye absorption.
Some dyes are anionic in water. In an immersion dye context, there is some electrostatic repulsion between the fabric and dye. (Ever rubbed balloons on your head to negatively charge them?) Salts are cationic, and help reduce the electronegativity of the fabric.As for the choice of table salt, it is simply a nontoxic and common household ingredient that happens to work well. [doi:10.1038/s41598–018–31501–7]However, if there is too high a ratio of salt to water, as in low-immersion dyeing to deliberately achieve unexpected patterns, salt can have a very different effect by reducing the dye’s solubility.
Adding dye will only make your colors darker. In other words, you cannot use dye to switch from a true blue to a true red, but you can add red to change a true blue to a dark purple. More on colors in a later section.
Additional sources / further reading
You can use color removers to uniformly get to a lighter color, or to selectively remove existing color in patterns (examples: shibori, bleach splatters, stonewash).
For serious, all-over color removal, you must either be willing to incur some chemical damage, or to subject your fabric to boiling temperatures, which may also damage it depending on the fiber content.
- Based on how they’re manufactured, some commercial fabrics can be difficult or impossible to remove the color from. You won’t know until you try.
- If the color is removable, you probably won’t be able to strip color all the way back to pure white. This can be fine if you plan to dye afterward or if you’re happy with off-white.
- The color results will be unpredictable unless you have prior experience with this exact fabric/dye/remover combo.
- All color removal processes will damage spandex. Anecdotally, you can get away with your fabric intact if your material has a spandex content under 5%, but spandex heat damage will still change the shape and drape of the fabric, so you might not want to use it on existing clothes that fit you really well. 
Home-accessible color removers
- Chlorine bleach: Often damaging to fabric.
- RIT Color Remover (sodium hydrosulfite), Thiox (thiourea dioxide): both require near-boiling temperatures. Overall, supposed to be less damaging than bleach, but note that boiling will damage spandex and shrink many natural fibers. (Check if your garment is pre-shrunk.)
I also read that a “stonewash” effect was traditionally achieved by literally adding stones (specifically, pumice marinated in an oxidizer), but can be achieved more gently using perlite!
Additional sources / further reading
Materials & methods
Once you have picked the appropriate chemicals, you will want to find some physical materials.
Gloves and a face mask, of course.
For laundry dyeing
Many of these hobbyist dyes are designed to be used in a top-loading laundry machine, but it can be hard to completely clean dye out of a laundry machine. It’s up to you whether you want to risk color bleeding into your future laundry (and your housemates’).
For immersion dyeing
You’ll want a big plastic bucket and a stirring stick. I use a 5-gallon bucket since my fiber-reactive dye recommends 3 gallons per pound of clothes plus some room to spare you from sloshing.
Budget around 2 hours for on-off stirring and washing out as much dye as you can by hand before you throw your fabric in the washing machine.
For stovetop dyeing
If your procedure requires high heat, don’t use your existing cookware; the manufacturers claim that the chemicals used will render your pots non-food-safe. Go to a thrift store and find a big cheap stockpot that you can ruin with crafts and keep separate from your cooking pots.
(FINALLY, THE FUN PART)
Quick review of color theory
Dyes are “subtractive colors”. This means that adding new colors will only make things darker. (“Additive color” refers to adding light.) As a simple example, you cannot use dye to switch from a true blue fabric to a true red. If your base color is blue, the closest to red you can get is purple, because the blue is a component that still exists within the color you see.
To approximate what hue you will end up with, look at a color wheel.
The hue you end up with will be roughly the average of your base color and your dye color, give or take some material-based caveats. If you start with blue and add red, you’ll get dark purple, but you’ll never be able to get all the way to true red because the underlying blue is still part of the color. If you start with red and add yellow, you’ll get orange, but probably a reddish-orange, since yellow dyes/solids are weaker than other colors. And whenever you add exact opposites on the color wheel, you usually get some kind of muddy brownish hue.
Want a better color prediction?
To more precisely pick dye colors and predict possible end results, go to your image editor of choice which has blend mode functionality. I use Photoshop. If you don’t already have an editor, GIMP and Inkscape are two good free options. Ideally, calibrate your monitor and make sure you have any display color temperature adjusters (such as Flux) turned off.
- Make a base layer that matches your existing fabric color as closely as possible.
- Add a layer on top with a dye color you want to use, and set the blend mode to “Multiply”.
- Tweak the dye layer using Saturation adjustments or the color picker until the end result matches your desired color. It may not be possible to get the color you want without first removing some base color.
Keep in mind that fabric doesn’t just appear as a solid color; it has many shades of light and dark, and looks different under different lighting conditions. For example, under yellow light, we might get something like this:
Calibrating the physical process
If you’re (1) dyeing something black, (2) you don’t care if the color gets dark so long as the hue is true, (3) or it turns out that the exact shade you need exists as a pre-made dye, then you’re in luck! You can just use the recommended quantity of dye for the recommended amount of time.
If not — for example, if you want to use a very subtle pastel tweak to warm or cool a color a little bit without over-darkening it (see this blog post), and you’re picky enough about colors that you’re willing to put in an extra few hours of work — you should probably do a test run on a scrap of as similar a fabric as you can find. Or if you’re removing color, because that’s really unpredictable.
Hobbyist dye instructions usually assume one pound of fabric, so weigh your fabric and multiply accordingly for the amount of fabric you’re going to dye.
Different dye colors, even within the same product line, may need more or less dye powder for the same amount of fabric. And as any bread baker knows, the same mass of something may be fluffier or denser depending on the formulation and the way you scooped it. If you need precise reproducible results between multiple dye batches, measure dye by weight instead of volume. But for most experimental hobbyist dyeing purposes, this level of precision doesn’t really matter.
If you are an aesthetic control freak like me (why else would you be here to painstakingly tweak your clothing colors to your exact specifications instead of getting different clothes or just following the brief instructions on the back of the dye kit box?), you may not be able to find the precise dye color you want.
I stumbled upon this amazing dye ratio resource by Tien Chiu, a textile artist who spent a year painstakingly creating an RGB cube of 250 different ratios of 6 different sets of Procion MX red-yellow-blue groupings and presenting the annotated, individually dyed skeins side by side so that you can triangulate the right color ratio for you. Wow. Thank you so much for being obsessive enough to bring this into reality.
Standard disclaimers that dye will be different on every material, and that it’s impossible to get a perfect color representation on a computer screen, so these results won’t match your results exactly. But also, it’s nicer to have some idea of what you’re getting into than to flail around blindly with no information whatsoever. For example, we can see from this grid how much more sensitive dye color ratios are to blue than to yellow — adding 0.5 parts blue dye significantly mutes color, but doubling yellow from 2 to 4 results in a barely perceptible change.
Enjoy your enablement into the exciting world of dyeing your clothes closer to the colors you love, and I hope you have fun!