diff --git a/src/util/color.c b/src/util/color.c
index a9f258f9..a7c33cb5 100644
--- a/src/util/color.c
+++ b/src/util/color.c
@@ -80,7 +80,7 @@ free_colors_lookup(void)
 }
 
 int
-decode_color(unsigned char *str, int slen, color_T *color)
+decode_color(const unsigned char *str, int slen, color_T *color)
 {
 	if (*str == '#' && (slen == 7 || slen == 4)) {
 		unsigned char buffer[7];
diff --git a/src/util/color.h b/src/util/color.h
index 4c2db0de..c1e153c0 100644
--- a/src/util/color.h
+++ b/src/util/color.h
@@ -13,7 +13,7 @@ struct color_pair {
 /* Decode the color string. */
 /* The color string can either contain '#FF0044' style declarations or
  * color names. */
-int decode_color(unsigned char *str, int slen, color_T *color);
+int decode_color(const unsigned char *str, int slen, color_T *color);
 
 /* Returns a string containing the color info. If no ``English'' name can be
  * found the hex color (#rrggbb) is returned in the given buffer. */