diff --git a/colout/colout.py b/colout/colout.py
index 3d3ebc0..293ed2c 100755
--- a/colout/colout.py
+++ b/colout/colout.py
@@ -391,7 +391,7 @@ def color_scale( name, text ):
 
     # normalize and scale over the nb of colors in cmap
     colormap = context["colormaps"][name]
-    i = int( math.ceil( (f - context["scale"][0]) / (context["scale"][1]-context["scale"][0]) * (len(colormap)-1) ) )
+    i = int( math.ceil( (f - context["scale"][0]) / (context["scale"][1]-context["scale"][0]) * len(colormap) ) ) - 1
     color = colormap[i]
 
     # infer mode from the color in the colormap