How to paint on textures?

I’m about to construct a robot that shall spray-paint an objekt. How can I do this in the best possible way? Open for any ideas.

/ Martin

Do a second pass with the paint texture.

Allocate some sysmem for the texture, draw on it when needed and then upload the texture to OpenGL, with glTexSubImage.
You can also just paint directly on the robot texture, but the other method allows for better texture sharing among other similiar robots.