diff --git a/tests/color_util.c b/tests/color_util.c
index 4b1471d6..da2f5c6b 100644
--- a/tests/color_util.c
+++ b/tests/color_util.c
@@ -390,6 +390,11 @@ scalar_stat_update(struct scalar_stat *stat,
 
 	stat->sum += val;
 	stat->count++;
+
+	if (stat->dump) {
+		fprintf(stat->dump, "%.8g %.5g %.5g %.5g %.5g\n",
+			val, pos->r, pos->g, pos->b, pos->a);
+	}
 }
 
 float
diff --git a/tests/color_util.h b/tests/color_util.h
index 724ff04a..1bb7297a 100644
--- a/tests/color_util.h
+++ b/tests/color_util.h
@@ -28,6 +28,7 @@
 
 #include <stdint.h>
 #include <stdbool.h>
+#include <stdio.h>
 
 enum color_chan_index {
 	COLOR_CHAN_R = 0,
@@ -131,6 +132,17 @@ struct scalar_stat {
 
 	double sum;
 	unsigned count;
+
+	/** Debug dump into file
+	 *
+	 * Initialize this to a writable file to get a record of all values
+	 * ever fed through this statistics accumulator. The file shall be
+	 * text with one value and its position per line:
+	 *   val pos.r pos.g pos.b pos.a
+	 *
+	 * Set to NULL to not record.
+	 */
+	FILE *dump;
 };
 
 struct rgb_diff_stat {
diff --git a/tests/weston-test-client-helper.c b/tests/weston-test-client-helper.c
index 89713e17..ca49a04d 100644
--- a/tests/weston-test-client-helper.c
+++ b/tests/weston-test-client-helper.c
@@ -38,6 +38,7 @@
 
 #include "test-config.h"
 #include "shared/os-compatibility.h"
+#include "shared/string-helpers.h"
 #include "shared/xalloc.h"
 #include <libweston/zalloc.h>
 #include "weston-test-client-helper.h"
@@ -1144,6 +1145,36 @@ image_filename(const char *basename)
 	return filename;
 }
 
+/** Open a writable file
+ *
+ * \param suffix Custom file name suffix.
+ * \return FILE pointer, or NULL on failure.
+ *
+ * The file name consists of output path, test name, and the given suffix.
+ * If environment variable WESTON_TEST_OUTPUT_PATH is set, it is used as the
+ * directory path, otherwise the current directory is used.
+ *
+ * The file will be writable. If it exists, it is truncated, otherwise it is
+ * created. Failures are logged.
+ */
+FILE *
+fopen_dump_file(const char *suffix)
+{
+	char *fname;
+	FILE *fp;
+
+	str_printf(&fname, "%s/%s-%s.txt", output_path(),
+		   get_test_name(), suffix);
+	fp = fopen(fname, "w");
+	if (!fp) {
+		testlog("Error: failed to open file '%s' for writing: %s\n",
+			fname, strerror(errno));
+	}
+	free(fname);
+
+	return fp;
+}
+
 struct format_map_entry {
 	cairo_format_t cairo;
 	pixman_format_code_t pixman;
diff --git a/tests/weston-test-client-helper.h b/tests/weston-test-client-helper.h
index dd20d514..34183249 100644
--- a/tests/weston-test-client-helper.h
+++ b/tests/weston-test-client-helper.h
@@ -250,6 +250,9 @@ screenshot_reference_filename(const char *basename, uint32_t seq);
 char *
 image_filename(const char *basename);
 
+FILE *
+fopen_dump_file(const char *suffix);
+
 bool
 check_images_match(pixman_image_t *img_a, pixman_image_t *img_b,
 		   const struct rectangle *clip,