Not a KiCad user, but assuming it works the same way as everywhere else, the mask layer is negative so if you draw things there, the mask is removed in that location. This gives you a palette of five different “colors”:
| Layers | Effect |
|---|---|
| (nothing) | Solder mask on epoxy laminate (some mask colors work better than others). |
| Copper | Solder mask on copper. |
| Copper+Mask | Plated copper, usually HASL (tin) or ENIG (gold on nickel). Avoid HASL with lead if you expect people to touch the surface. Nickel allergy is also a thing. |
| Mask | Bare epoxy laminate. |
| Silkscreen | Silkscreen (on top of any of the other combinations). |
There’s a bunch of videos on YouTube on doing 5-color artwork this way; e.g. here.
EDIT: Examples (rendered with EasyEDA’s camera mode):
