diff --git a/make.mk b/make.mk
index a83108bd..31189c8f 100644
--- a/make.mk
+++ b/make.mk
@@ -207,6 +207,10 @@ ifeq ($(LED), BLUE)
 CFLAGS += -DWATCH_IS_BLUE_BOARD
 endif
 
+ifndef COLOR
+$(error Set the COLOR variable to RED, BLUE, or GREEN depending on what board you have.)
+endif
+
 ifeq ($(COLOR), BLUE)
 CFLAGS += -DWATCH_IS_BLUE_BOARD
 endif