What pigments can paint on silica gel?

Acrylic pigment itself has strong adhesion. Maybe you made it too thick when you painted it, and it fell off when it dried. Try thinning it.

If it fails, it is recommended to use an oily marker. Even if you apply it very thick, you won't worry about falling off, but it may fade gradually after a long time. Conditional can be deepened on the original basis.

Go to the mobile phone beauty salon and ask, they should have better suggestions.