I had been using watercolors, but have noticed that it rubs off a bit, so I tried these:

Crayons that work like watercolors, the colors are bright and don't come off!

I first carefully color white tissue paper, I tend to use more than one color to get the right shade.

After you go over the color with a brush dipped in water, it looks like this.

Once it's dry you can start using it.