A method I often use is to color fabric material such as that with permanent marker. You'll have to color over it 2-3 times but it will hold. Things to know:
- Use a chisel tip. It's easier and faster.
- It will stink but when it's all complete, a few days in open air, the smell will go away. It eventually goes away completely.
- Doesn't dry as fast as you think, so watch your fingers. Good rule of thumb is as the smell dissipates, it's drying.
- Depending on the material it will or won't bleed through slightly.