Painting an STL file
It is indeed more related to the blender forum  than the 3d printing one. 

You can start by looking at this post from blender.stackexchange.com which explain how to unwrap and paint on a 3d object. You can also paint the texture on a 2d software like Gimp and then put it in the UV Image editor of Blender. Then you should be able to export your project (with the texture) in a suitable format for 3d print.