yea, I think it's a blue neon too.
There's some variables in how much color male peacocks have.
1. Are there other males in the tank? Are they bigger? Bigger males can suppress color in smaller males sometimes. The larger the tank, the more likely it is the smaller males will have some color. Also, some large males are pretty mellow. Others will chase colored up smaller males.
If the larger males are more aggressive, the smaller fish will probably show less color.
2. It varies by the individual. I recently bought a group of 6 small Eureka Reds from a breeder. One was colored up at about 3/4". I moved him to a different tank, hoping more males would color up. Still waiting.. I definitely have at least 2 more males in the group, but they aren't nearly as colored as the 3/4" one was when I first got it. In my own tank, I have a blue neon, about maybe 3" that has more color than the picture you attached. However, he grew up in a tank where he was the only male (according to the person I bought from). That might have something to do with it. He doesn't seem intimidated by the larger peacocks he shares a tank with now.
3. Not 100% sure on this.. I wonder if the presence of females helps the males color up more? I have heard this claimed, but I have no proof.
4. In general, you are right, the larger they are, the more colored they get. I bought a 3" Benga at the pet store. He looked awesome at the store. I put him in my community, and he turned almost completely grey (very faint yellow). Once he got 5", he started showing color again. He probably felt more comfortable as one of the "big boys".