Is there something I can do to keep my dummy cakes from fading in the sun? It only happens to the fondant but they have only been in the display window fora week? Thanks for any help.

I think some gel colours fade a lot faster than others - I've discovered that purple is terrible for fading. I'm wondering if you could colour the fondant for your display cakes with petal dusts/powder colour? - maybe that wouldn't fade?

Add 1 tsp of baking soda per pound of purple or pink coloured fondant to slow down the fading. Also dusting flowers will definitely slow down fading.

Sorry to be the one to tell you, but they are going to fade and there is nothing you can do about it. Putting a UV film on your windows will help. You can get it at Home Depot. I did make one grooms cake dummy that I used chocolate fondant to make all different shades of tans and browns. That cake only faded slightly. Best of luck.

Even if they aren't in a display window, they'll still fade under flourescent lighting (especially pinks and purples). Usually, Royal Icing doesn't fade as fast.

My deep pink and purple flowers are still quite deep in colour 1 year after adding the baking soda.

I covered one dummy in buttercream the turquoise and white one in my album and That one hasn't faded at all. Maybe I will do the colored cakes in BC and leave the white ones in fondant.

You can revive your color a little buy hand rubbing shortening over the fondant. Just put a little shortening in your hand and use one or two of your fingers to rub it in. As long as the fondant you are rubbing it into isn't really detailed work.
Another thing that works for me on smaller cakes is that I cover them with a glass dome. The glass makes kind of an airtight seal and holds in the color. I'd have about 20 smallish cakes under glass for about 1 year. I use them to decorate with in my kitchen. (One at a time at different times of the year). The fondant under glass does emit (sp) some kind of gas though and every now and then I have to lift the glass dome and spray glass cleaner inside then wipe it sparkley clean. I'll see if I can post a couple of pics. *Although you are probably talking about larger wedding style cakes.

...sorry picture didn't post

...one more try

Is it difficult to see through the window with the UV film on the window?

Most shops that frame artwork sell the UV-protective plastic.  Ask them to order you a few full sheets and build a display case with that instead of regular glass.