Update MONSTERS.md (#39276)

doc/MONSTERS.md

@@ -78,7 +78,7 @@ List of scenttype_id ignored by this monster. scent_types are defined in scent_t
 ## "symbol", "color"
 (string)
 
-Symbol and color representing monster in-game. The symbol must be a UTF-8 string, that is exactly one console cell width (may be several Unicode characters). The color must be a valid color id, see TODO: find or create a list of color ids.
+Symbol and color representing monster in-game. The symbol must be a UTF-8 string, that is exactly one console cell width (may be several Unicode characters). See [COLOR.md](https://github.com/CleverRaven/Cataclysm-DDA/blob/master/doc/COLOR.md) for details.
 
 ## "size"
 (string, optional)