Here's an eight-slot paint palette that fits into an Altoids Smalls tin. You'll have to squeeze it into the tin, but once it's in it'll slide around a bit. I used some blue tack (that poster hanging gum) underneath the insert to hold it in place. You'll also want to spray paint the inside of the tin's lid with white paint (and then maybe some polyurethane) to use as a paint mixing area.
Eight colors produces a lot of possible variations!
Once printed you'll likely have to spray it with polyurethane to make it waterproof. Otherwise your paints may leak.