I have a 6 inch male and he barely has any color besides a faint red in his tail......do they get their colors at 7-8"?
Most don't have much color, even as adults.
Because in nature Vieja maculicauda are predominantly fruit and algae eaters, they sometimes venture into the sea to graze on iodine and other vitamin rich growth, diet may be a very important factor in coloration.
If it were me, I'd be feeding an herbivore pellet, rich in marine algae such as NLS Algaemax, or another similar product.
Even some of the pellets used for color enhancement of koi might work well.