Adding gray layers in Color mode

I am trying to make a product look like a contoured part is different colors while retaining the contours. I can do most other colors, but any color like white, gray, black, etc. all look the same. I cannot get a dark gray while preserving the contours. I have tried changing color mode to grayscale and back to RGB, I have tried the hue/sat settings, and using a brush covers the contours. I like the Color mode since it is more realistic. I am using PS 25.3.1 on a PC. Any ideas? TIA.