How can I set the material of inside of an object?

I would like to set the material for the inside of a sphere. This is to create a custom skybox. How can i do this? Thank

You would have to flip the normals inside out, but from what I understand it's not recommended to use a model for a skybox since you shouldn't be able to physically 'reach' the sky anyway. You can just create your own skybox textures and use them to create a skybox material. I have read that skyboxes take 6 draw calls which may not be preferable on something like the iPhone, so in this case it might be beneficial to go with a model instead but I can't vouch for that approach. You'd be adding more polys anyway.

what about disabling backface culling and do two sided lighting as in opengl or dx why can’t unit do that ?