Color a texture dynamically, best method?

I have a single 2D texture that I want the player to be able to "paint" with their fingers. The texture would be black and white, and painting on it would reveal its original color. (Target platform: iOS)

I'm trying to find out the best way to achieve the effect (and many methods come to mind):

1. A black and white sprite on top of the color sprite. Somehow "erase" pixels from the black and white version's texture, revealing the color sprite that is underneath. I read that SetPixel is slow?

2. Modify vertex color or alpha to achieve same.

3. Delete vertexes from the top sprite, revealing the color one underneath.

4. Somehow drawing a mask??

Thank you for your insight. The solution must be performant on mobile (iOS).