I cannot find anywhere that explains how to paint the interior foundation. I'm not referring to placing the brick foundation inside of the home, but the intended foundation to create split levels and such. I'm so sick of looking at the pine colored base and I cannot for the life of me figure out how to paint it, hide it, cover it, whatever. I can't find a tutorial addressing my issue, either.

Hmm, it colors like any other colorable object. I've never had much of a problem coloring Interior Foundations. A picture showing your situation would help.
The only time it's become a bit of a pain, is when I put the walls of a house all the way down, which I rarely do, at which time the "pine colored base" of Interior Foundations become visible, which is a bit distracting, since it won't match colors of the surrounding walls. What I have to do in many cases like that is delete a nearby wall segment and use Shift+click to color in all the Interior Foundations before replacing the wall segment. Sounds more complicated than it is.

Give me a little but and I will take a screen shot. I'm uploading some CC.

You can paint the stage foundation so long as nothing is touching it. Or sometimes if just one side is touched it will or it wont, it's finicky like that. So you place it then you paint it. If you put stairs to it, or a wall or a bit of edge fencing it may or may not stop being recolourable. If you have an edge fence around the top because you are making say a raised garden it wont recolour at all. So you would have to remove one side of the edging and test, and keep removing if its being suborn. If you can remember to paint it as soon as you place it you should not have an issue.

I got it figured out, thanks guys. It was the fact that I kept trying to paint it AFTER I had attached stairs. I always build all the architecture then go back and do all my painting.