I did some research about this same topic, when I got my white Sultan, Diana..
I've read that chickens who are fed a lot of corn tend to take on a yellowish, brassy color, but I don't know what you are feeding them. As for sun protection, maybe you can give them access to some shade. Natural shade, like trees or shrubs are good, but I also made shady areas for my ducks and chickens by leaning a few planks up against their wire run. The birds are good about staying out of the sun if they have the option too, but don't expect them to give up sun-bathing!
Also, a few days before your show, definatly give them a bath! It helps get dirt out of their feathers, and makes them fluffy. But don't bathe them too close to the show, or the chickens won't be able to preen their natural oils back into their feathers.
There is shampoo that I used, meant for white horses and dogs, that has a 'blueing' substance that is good for white feathers. I bought it for our horses, but it works wonderfully on birds too- it made my WCB Polish have lovely white crests! You only want to use very small amounts of that shampoo, because it will tint their feathers an undesirable blue tint!
