Clarify sorting order of RGB colors

man/rgbgfx.1

@@ -421,7 +421,7 @@ be
 .Em exactly
 the same.
 .It
-If none of the above apply, colors are sorted from lightest to darkest.
+If none of the above apply, colors are sorted from lightest (first) to darkest (last).
 The definition of luminance that
 .Nm
 uses is