When we talk about the rainbow trout, the main cause of different skin color can totally be depended upon the genetic differences as well as strains.
Description about darkness of rainbow trout
The rainbow varies a lot, but you usually see some sort of stripe, it could be an incredible red slash. Some are very silvery, some are darker, could depend on diet and their home water.
All are beautiful fish. They could be more heavily spotted like here, with a stripe that is more purplish. Also one needs to understand that if the density of stock in fish increases in a particular place, then these rainbow trout become more dark or darkest.