I always see these cake decorators using powered sugar, or cornstarch, when rolling out fondant. My question is, how do they clean this off the fondant after decorating? Specially on black or dark colored decorations??

Good question - I have wondered that myself.

You can brush it with water or vanilla. The vanilla leaves more of a sheen so either use it sparingly or brush the entire surface so it will be even.

oh thats how they get the shine?

You can take a piece of fondant that doesn't have cornstarch or powdered sugar on it and rub it on the cake surface. This will clean up the cake without making it wet or giving it a shine.
If you want a shine on your cake you can steam it like they do on Cake Boss.

If you steam the fondant cake it will clean the sugars and powders off PLUS will give a really nice sheen. Just be careful not to steam too long it will melt the fondant.

I rub my cakes with shortening to clean them. It works two fold: I moisturize the fondant to keep it from cracking and it gives it a nice shine.

I've also used a damp cloth to polish it off.

For dark fondants, I once had a pro pastry chef tell me to use cocoa powder instead of powdered sugar when rolling out.

If it is on dark fondants I use a little alcohol that seems to work really well. I dust as much off as possible firstly with a clean paint brush then use a little alcohol or rejuvenator spirit on a brush it seems to dissolve the powdered sugar or cornstarch but not affect the fondant. HTH

I try not to overdo the powdered sugar to avoid the look but I have a large pastry brush that I use to brush off excess powdered sugar. It has very fine bristles and works really well to remove the powder.

thanks to everyone for the info!!