diff --git a/src/util/color.c b/src/util/color.c
index 5b7872a59..d6684ebbb 100644
--- a/src/util/color.c
+++ b/src/util/color.c
@@ -82,7 +82,7 @@ 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 80ad9f8b7..52f7353c6 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. */