I did some experiments a while back to find what palette I could get using all combinations of mask/silk/copper. I found silk+copper(-mask) gives a slightly orange tinted white, and silk(-mask-copper) gives a slightly whiter white than silk+mask(-copper) on its own. Both of these looked flatter as well, as the solder mask adds an extra bit of height. Unfortunately silk alignment isn’t always super precise, so it’s more suited to larger things. The effect is quite subtle, really only visible when next to other uses of silk.
There’s also exposing FR4 on both sides of the board, or exposing FR4 on one side, and silk or mask on the other, for varying degrees of “transparency”. I have plans to use exposed FR4 as a light diffuser for an upcoming project.