Check that color is valid

Instead of merely checking that COLOR is set, check that it is one of
RED, BLUE or GREEN

make.mk

@@ -213,6 +213,12 @@ ifndef COLOR
 $(error Set the COLOR variable to RED, BLUE, or GREEN depending on what board you have.)
 endif
 
+COLOR_VALID := $(filter $(COLOR),RED BLUE GREEN)
+
+ifeq ($(COLOR_VALID),)
+$(error COLOR must be RED, BLUE, or GREEN)
+endif
+
 ifeq ($(COLOR), BLUE)
 CFLAGS += -DWATCH_IS_BLUE_BOARD
 endif