Synthetic hair can't be dyed with traditional hair dyes because it isn't porous, but it is still possible to alter the color of it provided you can get something to stick to the outside of the strands. Enter: acrylic ink!
To color synthetic hair, mix acrylic ink with 70% isopropyl rubbing alcohol. Less ink will result in fainter, paler colors, and more ink will be result in richer, more vibrant colors.
Lay the hair flat out on a protected surface, like a few layers of newspaper or an old towel.
Put the ink mixture in a spray bottle and put on some protective gloves, then spray the ink gently onto the hair.
Leave the hair overnight or longer so that it's completely dry. You can also use a hair dryer, but that can get messy; we recommend planning ahead so you can let it dry naturally :)
Once the ink is completely dry, rinse the hair in cold water and let it dry completely again before use.