I'm already thinking ahead to Halloween, and I'd like to make the wilton Mummy head for a neighbor of mine, and I'd like to put the cake on a cake board covered in marbelized black and orange fondant.
Will this combination cause the orange to become discolored?
I would do some test runs. Make the fondant, and use a little at a time to see if you need resting time, if the colours will stay separated, and if not, how long the colours stay separated. Personally, I would use pre-coloured fondant (I've already got tons of black from another project) - I think it will keep its colour better. I've twisted black and white together and the black it didn't bleed at all. Adding some Gem Tex (or whatever it is you use) will help secure the colour, I think.
And this sounds like a great idea. I just might have to steal it!
I've never had a problem with dark and light colors together. I roll logs of each color, lay them side by side in about 4-6 strips of each then begin to roll those into a ball, gently pressing together. Works out great!
just be careful not to over marble - if you need to roll it out more than once, it will get murky very quickly! seems more noticeable than with other colours, cause the black over rides rather than mixes ( i guess?).
roll out black fondant then add balls and thick strips of orange fondant and roll again if not marbled enough just roll again till desired marble effect you want and don't over do it as brincess_b said, that is how I do clay