diff --git a/tests/color_util.c b/tests/color_util.c
index acbd040a..4b1471d6 100644
--- a/tests/color_util.c
+++ b/tests/color_util.c
@@ -374,7 +374,9 @@ lcmsMAT3_invert(struct lcmsMAT3 *result, const struct lcmsMAT3 *mat)
 }
 
 void
-scalar_stat_update(struct scalar_stat *stat, double val, struct color_float *pos)
+scalar_stat_update(struct scalar_stat *stat,
+		   double val,
+		   const struct color_float *pos)
 {
 	if (stat->count == 0 || stat->min > val) {
 		stat->min = val;
@@ -441,7 +443,8 @@ rgb_diff_stat_print(const struct rgb_diff_stat *stat,
 
 void
 rgb_diff_stat_update(struct rgb_diff_stat *stat,
-		     struct color_float *ref, struct color_float *val)
+		     const struct color_float *ref,
+		     const struct color_float *val)
 {
 	unsigned i;
 	double ssd = 0.0;
diff --git a/tests/color_util.h b/tests/color_util.h
index 2278c874..724ff04a 100644
--- a/tests/color_util.h
+++ b/tests/color_util.h
@@ -139,7 +139,9 @@ struct rgb_diff_stat {
 };
 
 void
-scalar_stat_update(struct scalar_stat *stat, double val, struct color_float *pos);
+scalar_stat_update(struct scalar_stat *stat,
+		   double val,
+		   const struct color_float *pos);
 
 float
 scalar_stat_avg(const struct scalar_stat *stat);
@@ -149,7 +151,8 @@ scalar_stat_print_float(const struct scalar_stat *stat);
 
 void
 rgb_diff_stat_update(struct rgb_diff_stat *stat,
-		     struct color_float *ref, struct color_float *val);
+		     const struct color_float *ref,
+		     const struct color_float *val);
 
 void
 rgb_diff_stat_print(const struct rgb_diff_stat *stat,