diff --git a/src/brainbrowser/lib/color-map.js b/src/brainbrowser/lib/color-map.js
index a79bc62e..9742e249 100644
--- a/src/brainbrowser/lib/color-map.js
+++ b/src/brainbrowser/lib/color-map.js
@@ -76,6 +76,11 @@
         if (line_length < 3) continue;
 
         if (line_length > 4) {
+          /* Sparse colour map. The first column gives the
+           * label to associate with the colour, the remaining
+           * 4 columns give the RGBA values to associate with
+           * the label.
+           */
           ic = parseInt(color[0], 10);
           ic *= 4;
           line_length = 4;