how should I change it?

it depends, what was the denial?

It looks very flat to be honest. I think people would be able to help you better if you put the wear on the HA?

did they say item looks gray possibly due to black shading or white highlights? Or something about it resembling old site art? I had to get through about five denials before I figured out what the heck they were talking about XP

Gotta add more layers of shadows (: I see a highlight, the base color, a shadow, and a very faint darker shadow. The darker shadow needs to be darker xP For the blue parts, make the shadows dark blue, closer to purple, and for the yellow parts, make the shadows more brown. You need a darker shadow layer to go inside the shadows you already have. Think of it as defining the shadows more. That's my advice xD