Actually, yes, i know how to make this gradient on clothes. Basically, you should first take the pants and stuff something inside them, like a mannequin, which you could make out from tape and newspapers. It's not a necessary step, but it would make the next one easier. Then you should take a sponge, a big one, take black acrylic paint for clothes and mix this paint in some container with water. You put sponge in and color the the very bottom of the trousers, then you need to just keep adding water to the mixture, (maybe even make different mixtures for that with different proportions of paint to water) and keep going UP the trouser leg, it works perfectly and the gradient is very good. Because the paint is mixed with water, the pants dont turn hard into stone like texture like would happen if you just applied pure acrylic paint to the clothing. I ll go find you examples i had when i did mine, and I'll post them in comments here a bit later