Recently I started working with Kicad 9.0, and some settings were changed, among those settings and when I didn’t know how to change the color of the board in 3D viewer, so for the poeple who are interested in knowning that, here is how to do it :
In PCB editor go to “File” → “Board setup” → under “Board Stackup” click on “Physical Stackup” from here you can select the colors you want for the Top solder mask and bottom solder mask.
Finally in 3D viewer, don’t forget to enable “Use board Stackup colors”.