I created a cube and applied a color to it. I then saved it as .blend and loaded it again to check and all OK.
I exported it as a obj and loaded it again and it worked.
I loaded the obj in prefab3d and it loaded the cube but not the color?
When I export a shape with materials/texures (obj) do I need to set any setting so another graphics rendering app (Prefab3d) can pick this up?
No, the .obj file will be exported with a .mtl file that contains the materials for your model. The .obj should point to this file when opened in your app, at least it works that way for the apps that I have used.