Has anyone put icing over ganashe?

Yes, I have. I did a messy buttercream finish with the blue coloured showing through after I scraped some of the bc icing off. 

I'm making a ti my little Pony cake for my daughter's birthday and she wants a semi sweet ganache with a whipped icing over top colored blue green and yellow (each tier a different color) and I'm worried that either the ganashe will mix with the icing or it will show through

Firstly, your ganache will dry rock hard, if made correctly. But I must caution you, white ganache is very sweet. I wouldn't personally do it again. Why not use Royal Icing? It dries very hard, you can get a smooth finish. And when you cut your cake, royal icing will have softened nicely from the buttercream.

Hope this helps...

Yes I have lots of times. It works well for me- my ganache dries very hard and I usually pop that cake in the refrigerator for at least 15 minutes to really set the ganache and then go ahead and ice right over top of it. I have done it with american buttercream and whipped buttercream with good results. :-)