Hello. I am a loopy addict. I even modified the source code to set COL_SATISFIED to light grey...

I mean this:

$ git diff
diff --git a/loopy.c b/loopy.c
index 0a03cf4..51ac14b 100644
--- a/loopy.c
+++ b/loopy.c
@@ -850,9 +850,9 @@ static float *game_colours(frontend *fe, int *ncolours)
     ret[COL_MISTAKE * 3 + 1] = 0.0F;
     ret[COL_MISTAKE * 3 + 2] = 0.0F;
 
-    ret[COL_SATISFIED * 3 + 0] = 0.0F;
-    ret[COL_SATISFIED * 3 + 1] = 0.0F;
-    ret[COL_SATISFIED * 3 + 2] = 0.0F;
+    ret[COL_SATISFIED * 3 + 0] = 0.8F;
+    ret[COL_SATISFIED * 3 + 1] = 0.8F;
+    ret[COL_SATISFIED * 3 + 2] = 0.8F;
 
     /* We want the faint lines to be a bit darker than the background.
      * Except if the background is pretty dark already; then it ought to be a

You should be able to get the same effect by setting LOOPY_COLOUR_5=cccccc in your environment. (It's hacky, but it fills the gap until I get round to writing a proper user-preferences layer.)