Do you need to re-color anything? Create a hue/saturation layer and mask it.
A curves adjustment layer can add a LOT of color complexity and beauty to individual elements, or the whole piece.
Do you want to experiment with some changes, copy the flattened layer and make the changes, then compare with the original and save the better version.
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?
Need to create a color bridge to unify a complex palette? Draw a line (curved is fine) along the color palette, and pick colors along this line. This will create structure, a bridge of colors connecting the two most contrasted colors. Feel free to add a “special” color, an accent color, that break out of …
Do you need more color harmony? Try to get the colors to interact, to “kiss”. Bring some of one color into another. Desaturate to help them play better together. Build bridges between colors. Find a way to distribute a color throughout the piece, instead of just having it in one place.
Do you need to unify the colors? Fill a layer with color, set it to the “color” blending mode, and dial up the opacity until the colors unify.
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?