I think some pictures would be really helpful here.
As a general rule and depending on the strain that you are talking about, young discus can look pretty different from how they will end up at adults. But this really depends on the type. For example, a blue diamond isn't going to change from 2.5 inches to 6 inches, but a white butterfly might start out pink/orange and slowly get white as it matures. This trend is also true for many spotted fish.
What strains are you curious about?
Best of luck.