Inside an object

How do I texture the inside and outside of a mesh object?

Say I am making a sphere for atmosphere.
I want to have a transparent texture and display clouds outside.
and when the camera is inside I want to paint that sky blue.

Do I need two objects?

[Borrillis]

That's probably a bad idea. What you might want to consider is changing the skybox/spere/cube once you have closed the distance to your planet beyond some threshold. I.e. if your camera is > 8Km from the surface of a planet render a space skybox, and add a cloud layer to your planet. Once your camera is < 3Km from the surface stop layering the clouds on the planet and switch to a cloud skybox. Just my $.02.

sounds good
do you think it would be hard to create a dynamic materials for a skybox object?

I was thinking different colors of sky for different types of atmosphere.
and some blending (i.e darker colors directly overhead, and "whiter" at the horizon?

[Borrillis]

Take a look at the DynamicTextures sample.

I used this code to generate the sun and planet objects:
http://www.ogre3d.org/wiki/index.php/ManualSphereMeshes
I noticed at the top of the page that the original code had the sphere pointing inward.

What if I did an atmosphere as two of these one pointing inward one outward.
same position/size.