With wild trout, it depends a lot on the area you're fishing and where it is. "Natural" colors are always better.

With hatchery planted trout, they'll strike at pretty much anything shiny, either chrome-, nickel-, and gold-plated, or nickle and blue.

My personal favorite (when trout are planted in my local lake and the opportunity to fish for "bait" arises) is a gold- or silver-plated Acme Kastmaster from 1/8 to 1/2 ounce.