Need more color complexity? Use a saturation layer and target certain colors to adjust and refine! You can also check to see if it’s too monochromatic – if one channel changes most of the piece, that may be a red flag!
Do you need more of one color in another color? Add a small amount of one color in an area dominated by another color. This applies to more than color: Do you need to add A bit of cool color to warm colors? Warm color to cool colors? Light to your shadows?
When you’re close to finished, consider an airbrush with color dodge set to push the lights, bring in extra color to highlights. Copy-merge what you have (Command/Ctrl + Shift + c) Select a bright color and paint on the copy-merged layer. Toggle the visibility of the layer to see if it was an improvement.
Do you have enough detail in the light areas of the piece, or is the detail blown-out by too much brightness, and needs to be brought back in?
Do you need to copy merge to get a flat version? Make some changes to the flat version, mask some parts out, copy-merge again if you want.
Where’s the focal point? Can you increase the contrast, add a pop of white or the main color to draw attention to the focal point?
Any highlights that need punching? Think about the surfaces you’re communicating. Can you make the texture and surface clearer with highlights?
Does anything unnaturally stick out? Do you need to content-aware healing brush any weird spots?
What about interaction of colors? Is there any bounced light that needs to be painted in?
Highlights tell the story of the surface, do you need more detail in highlights? Shadowy areas can remain relatively un-detailed.