If you just bought it, it is likely stressed out and trying to get accustomed to its new environment. Give it a few days. Color often fades when the fish is freaked out.
Yes, peacocks do color as they grow. Some color very early while still small and others must be bigger before they show their actual brilliance. When they are real small, it is almost impossible tell males from females (for some species). At 2.5", you shouldn't have that problem.
Can you post a picture?