Yes the color will show thru onto the white , from a dark fondant background.
Put your piping gel on the fondant, then lay the piece onto the cake. Its works better this way., because if you put the piping gel on the image first, then it gets wet and messy and harder to handle.
I hope I have answered all question for you. If not ask some more or personnel email me. I am always glad to help. Plus I have used the system for 9 years and used it in alot of different ways and mediums.
If you go to my website, look under wedding cakes, look at the tatoo cake, that is a PhotoFrost image , cut out and laid on fondant, Happy Decorating.