Whenever Fritz finishes analyzing my game it gives a colored box. Either nothing, or half red half black, or rarely completely red. What does it mean? I think it's the mistake level but I'm not sure.

I have Fritz 12 and I'm not sure what you're talking about. Could you be a little more specific? Maybe you could cut and paste the analysis with the box in it and post it here? I haven't used F12 in awhile but I do recall the docs and help file do explain all the symbols and notations.

There are lots of YouTube tutorials on Fritz, look around for one that may apply to your question. Ultimately though you really should RTFM.

http://www.chessbase.com/newsdetail.asp?newsid=6105