When my sister gave me these unworn (she didn't like them for above reasons) uniqlo jeggings, I really wanted to keep them so some changes had to be made. By dip dying the top part of white jeans, you can avoid visible underwear, grass stains, and a color that makes your hips and butt look bigger - at least a little. There are tons of dye tutorials around for cotton jeans, but I couldn't find anything to help with these semi synthetic pants. I've tutorialized this slightly more complex process for others that want to dye their polyester pants as well.
You will need:
Jeans or jeggings in a white or a light color (uniqlo jeggings, cotton poly blend)
Jaquard iDye suited to the pants you are dying (I used the original iDye + iDye poly because these pants are a blend)
Water and any additives listed in the instructions on your dye.
A stainless steel or ceramic pot, it will NOT be safe for cooking after this.
A container to mix dye in, NOT for food. I mix dye in reusable squirt bottles.
A spoon or stirrer, NOT to be used for cooking, note that the spoon is labeled DYE.
A safety pin
A measuring tape
A hotplate, it is always safer to use chemical dyes outside of your kitchen to reduce the chance of contamination.
An extension cord.
A rig to hold your pants away from the heat of the burner, your arm will probably get tired.
A second pot or bowl to catch drips.
Before you start, it is very helpful to mark the place where you want the dye to stop. The transition of color will happen over 9-12", so keep that in mind. I used a measuring tape to find the height, then marked the spot with a safety pin because a marker or pen could leave a stain.
Submerge the pants in water and make sure they are completely soaked.
While the pants are soaking, open your dyes. Most dyes and brands have different instructions, so read the instructions before you continue. They usually dont say anything about ombre dying, but things like water temperature, added ingredients, and cooking time will be the same. My pants are a cotton poly blend, so I will be using two different dyes to cover all the fibers. The poly dye comes with an enhancer which helps the color soak into synthetics, and the cotton dye calls for salt.
Dissolve the dye in some water, ignoring the other ingredients. iDye comes in disolvable packages, but these are not going to help you ombre dye. I just opened them and poured the dye powder out. Mix the dye solution thoroughly. You will add this to the pot slowly and a squirt bottle helps control the flow of liquid as you go.
Remove the pants from the cooking pot and place them in a clean sink or bowl. Add the salt and dye enhancer or whatever materials your instructions call for to the water and turn on the heat.
While the water is heating, suspend your pants cuffs up above the pot. You can do this using a tree branch, a clothes line, or tripods if you have them. Keep in mind that you will need to be able to raise and lower the pants a few times to different heights. If your dye does not require heat, you can let the cuffs hang over the edge of the pot, but please do not do this on the hot plate, bad things will happen.
Add dye to the water until it looks black in the pot, and looks light grey when you dip and remove a paper towel from the water. The dunked towel test will show a color close to what your pants will look like.
Lower the pants into the hot dye bath up to the safety pin. They should stay here for half the time that is required in the instructions. For mine, I lef them for 1/2 hour, stirring periodically.
Remove the pants from the dye, add more dye slowly, testing the color with a paper towel. This time you'll want to see middle grey. Lower the pants back into the dye bath 3 inches less than last dipso that less leg should is submerged.again, cook for half the recommended time, and stir a few times.
Repeat this step 1-3 more times until the top of your pants is black. On the last dip, may want to leave the pants in for extra long to get a true black
Remove the pants from the dye keeping the white ankles above the dyed part at all times, flipping them over before rinsing will cause permanent black drip marks. Hose the pants down (not shown) and hang them to dry ankles up.
Like mine , your pants may not be 100% black, but they should be pretty dark. Polyester is not the easiest material to dye at home, and cotton will get a lot darker. That said, I no longer worry about all bad aspects of white jeggings, so I'm calling this a success.