Ok, so this is really two questions:
Can a cake be covered in white chocolate ganache the same way you would do so as buttercream? (and then decorated with fondant accents)
If so, can that ganache be colored? I want to get a dark blue color - whats the best way of getting that color?
All help is greatly appreciated, thanks in advance
I think you can do this. I remember reading some posts about using whipped ganache under fondant, so think you should whip it.
You probably should add the coloring to the cream before mixing into the chocolate. I would use candy coloring. It is oil based and won't make the chocolate seize up. Although come to think of it, if you add the coloring to the cream first it might not matter if you used regular gel coloring.
Here is a long post about using ganache on the cake. You may want to read it as it might have some good info and ideas.