diff --git a/app/core/gimphistogram.c b/app/core/gimphistogram.c
index 76c8e2b220..2e654d3f31 100644
--- a/app/core/gimphistogram.c
+++ b/app/core/gimphistogram.c
@@ -27,6 +27,7 @@
 #include "libgimpbase/gimpbase.h"
 #include "libgimpmath/gimpmath.h"
 #include "libgimpcolor/gimpcolor.h"
+#include "libgimpcolor/gimpcolor-private.h"
 
 #include "core-types.h"
 
diff --git a/app/core/gimpimage-convert-indexed.c b/app/core/gimpimage-convert-indexed.c
index 4dcf5d6d94..22e6d1f7bd 100644
--- a/app/core/gimpimage-convert-indexed.c
+++ b/app/core/gimpimage-convert-indexed.c
@@ -148,6 +148,7 @@
 #include <gegl.h>
 
 #include "libgimpcolor/gimpcolor.h"
+#include "libgimpcolor/gimpcolor-private.h"
 #include "libgimpmath/gimpmath.h"
 
 #include "core-types.h"
diff --git a/app/gui/splash.c b/app/gui/splash.c
index cf7638392d..eeb9958625 100644
--- a/app/gui/splash.c
+++ b/app/gui/splash.c
@@ -34,6 +34,7 @@
 #include "libgimpbase/gimpbase.h"
 #include "libgimpmath/gimpmath.h"
 #include "libgimpcolor/gimpcolor.h"
+#include "libgimpcolor/gimpcolor-private.h"
 #include "libgimpwidgets/gimpwidgets.h"
 
 #include "gui-types.h"
diff --git a/app/operations/gimplevelsconfig.c b/app/operations/gimplevelsconfig.c
index 57d5d91901..f8c7773b8f 100644
--- a/app/operations/gimplevelsconfig.c
+++ b/app/operations/gimplevelsconfig.c
@@ -28,6 +28,7 @@
 
 #include "libgimpbase/gimpbase.h"
 #include "libgimpcolor/gimpcolor.h"
+#include "libgimpcolor/gimpcolor-private.h"
 #include "libgimpmath/gimpmath.h"
 #include "libgimpconfig/gimpconfig.h"
 
diff --git a/app/operations/gimpoperationcolorize.c b/app/operations/gimpoperationcolorize.c
index b6a8a1b188..af3c41755a 100644
--- a/app/operations/gimpoperationcolorize.c
+++ b/app/operations/gimpoperationcolorize.c
@@ -25,6 +25,7 @@
 #include <gegl.h>
 
 #include "libgimpcolor/gimpcolor.h"
+#include "libgimpcolor/gimpcolor-private.h"
 #include "libgimpconfig/gimpconfig.h"
 
 #include "operations-types.h"
diff --git a/app/operations/gimpoperationthreshold.c b/app/operations/gimpoperationthreshold.c
index e1461b9d55..afbbc049f4 100644
--- a/app/operations/gimpoperationthreshold.c
+++ b/app/operations/gimpoperationthreshold.c
@@ -26,6 +26,7 @@
 
 #include "libgimpbase/gimpbase.h"
 #include "libgimpcolor/gimpcolor.h"
+#include "libgimpcolor/gimpcolor-private.h"
 #include "libgimpconfig/gimpconfig.h"
 
 #include "operations-types.h"
diff --git a/app/tools/gimpcurvestool.c b/app/tools/gimpcurvestool.c
index 266ff138f9..23ea73bd2c 100644
--- a/app/tools/gimpcurvestool.c
+++ b/app/tools/gimpcurvestool.c
@@ -25,6 +25,7 @@
 
 #include "libgimpbase/gimpbase.h"
 #include "libgimpcolor/gimpcolor.h"
+#include "libgimpcolor/gimpcolor-private.h"
 #include "libgimpconfig/gimpconfig.h"
 #include "libgimpwidgets/gimpwidgets.h"
 
diff --git a/app/widgets/gimpgradienteditor.c b/app/widgets/gimpgradienteditor.c
index 20c31196d5..c59afab04b 100644
--- a/app/widgets/gimpgradienteditor.c
+++ b/app/widgets/gimpgradienteditor.c
@@ -51,6 +51,7 @@
 #include <gtk/gtk.h>
 
 #include "libgimpcolor/gimpcolor.h"
+#include "libgimpcolor/gimpcolor-private.h"
 #include "libgimpmath/gimpmath.h"
 #include "libgimpbase/gimpbase.h"
 #include "libgimpwidgets/gimpwidgets.h"
diff --git a/libgimpcolor/gimpcolor-private.h b/libgimpcolor/gimpcolor-private.h
new file mode 100644
index 0000000000..9a9716d6ba
--- /dev/null
+++ b/libgimpcolor/gimpcolor-private.h
@@ -0,0 +1,40 @@
+/* LIBGIMP - The GIMP Library
+ * Copyright (C) 1995-1997 Peter Mattis and Spencer Kimball
+ *
+ * This library is free software: you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 3 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Library General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library.  If not, see
+ * <https://www.gnu.org/licenses/>.
+ */
+
+#ifndef __GIMP_COLOR_PRIVATE_H__
+#define __GIMP_COLOR_PRIVATE_H__
+
+/* Legacy definition to calculate luminance from sRGB.
+ *
+ * These values and macro are specific to a "RGB float" to "Y float"
+ * conversion within sRGB space. These should not be used in new code,
+ * where converting to "Y float", space aware, may often be what you
+ * wanted to do.
+ * Once we got rid of all remaining usages in plug-ins and app, we can
+ * delete this file as it's not distributed.
+ */
+#define GIMP_RGB_LUMINANCE_RED    (0.22248840)
+#define GIMP_RGB_LUMINANCE_GREEN  (0.71690369)
+#define GIMP_RGB_LUMINANCE_BLUE   (0.06060791)
+
+#define GIMP_RGB_LUMINANCE(r,g,b) ((r) * GIMP_RGB_LUMINANCE_RED   + \
+                                   (g) * GIMP_RGB_LUMINANCE_GREEN + \
+                                   (b) * GIMP_RGB_LUMINANCE_BLUE)
+
+
+#endif  /* __GIMP_COLOR_PRIVATE_H__ */
diff --git a/libgimpcolor/gimpcolor.h b/libgimpcolor/gimpcolor.h
index 327b47d1b3..5cb2e4d832 100644
--- a/libgimpcolor/gimpcolor.h
+++ b/libgimpcolor/gimpcolor.h
@@ -93,16 +93,6 @@ GParamSpec * gimp_param_spec_color_from_string (const gchar *name,
 gboolean     gimp_param_spec_color_has_alpha   (GParamSpec  *pspec);
 
 
-/* Legacy definition to calculate luminance from sRGB */
-#define GIMP_RGB_LUMINANCE_RED    (0.22248840)
-#define GIMP_RGB_LUMINANCE_GREEN  (0.71690369)
-#define GIMP_RGB_LUMINANCE_BLUE   (0.06060791)
-
-#define GIMP_RGB_LUMINANCE(r,g,b) ((r) * GIMP_RGB_LUMINANCE_RED   + \
-                                   (g) * GIMP_RGB_LUMINANCE_GREEN + \
-                                   (b) * GIMP_RGB_LUMINANCE_BLUE)
-
-
 
 /*
  * GIMP_TYPE_BABL_FORMAT
diff --git a/plug-ins/common/despeckle.c b/plug-ins/common/despeckle.c
index fdf19a75d5..3250925b94 100644
--- a/plug-ins/common/despeckle.c
+++ b/plug-ins/common/despeckle.c
@@ -24,6 +24,8 @@
 
 #include <stdlib.h>
 
+#include "libgimpcolor/gimpcolor-private.h"
+
 #include <libgimp/gimp.h>
 #include <libgimp/gimpui.h>
 
diff --git a/plug-ins/common/file-aa.c b/plug-ins/common/file-aa.c
index 56299cbee0..2e8b5fc1b2 100644
--- a/plug-ins/common/file-aa.c
+++ b/plug-ins/common/file-aa.c
@@ -31,6 +31,8 @@
 
 #include <aalib.h>
 
+#include "libgimpcolor/gimpcolor-private.h"
+
 #include <libgimp/gimp.h>
 #include <libgimp/gimpui.h>
 
diff --git a/plug-ins/common/file-pnm.c b/plug-ins/common/file-pnm.c
index 8299d9c2f5..397eee5b32 100644
--- a/plug-ins/common/file-pnm.c
+++ b/plug-ins/common/file-pnm.c
@@ -33,6 +33,8 @@
 #include <math.h>
 #include <errno.h>
 
+#include "libgimpcolor/gimpcolor-private.h"
+
 #include <libgimp/gimp.h>
 #include <libgimp/gimpui.h>
 
diff --git a/plug-ins/file-fli/fli-gimp.c b/plug-ins/file-fli/fli-gimp.c
index 60d35dd43c..d5ca360735 100644
--- a/plug-ins/file-fli/fli-gimp.c
+++ b/plug-ins/file-fli/fli-gimp.c
@@ -56,6 +56,8 @@
 
 #include <glib/gstdio.h>
 
+#include "libgimpcolor/gimpcolor-private.h"
+
 #include <libgimp/gimp.h>
 #include <libgimp/gimpui.h>
 
diff --git a/plug-ins/gimpressionist/gimp.c b/plug-ins/gimpressionist/gimp.c
index 2072dba5e9..102805fe03 100644
--- a/plug-ins/gimpressionist/gimp.c
+++ b/plug-ins/gimpressionist/gimp.c
@@ -21,6 +21,8 @@
 
 #include <gtk/gtk.h>
 
+#include "libgimpcolor/gimpcolor-private.h"
+
 #include <libgimp/gimp.h>
 
 #include "ppmtool.h"
diff --git a/plug-ins/gradient-flare/gradient-flare.c b/plug-ins/gradient-flare/gradient-flare.c
index 68bed7cab2..ed842e9adb 100644
--- a/plug-ins/gradient-flare/gradient-flare.c
+++ b/plug-ins/gradient-flare/gradient-flare.c
@@ -44,6 +44,8 @@
 
 #include <glib-object.h>
 
+#include "libgimpcolor/gimpcolor-private.h"
+
 #include <libgimp/gimp.h>
 #include <libgimp/gimpui.h>