Painting in game Help

Well I am not sure that I need ray. Because my game will be a fix screen. So player will not be able to look around. So giving a ray to where he looks will not help me.

Well I guess I need to make player to be able to choose meshes in game with a click. Then set material. Do you have any ideo how do i make user to be able to choose meshes in game?

Thanks for your help.