diff --git a/tests/belts.scad b/tests/belts.scad
index d7d8d40..3477a39 100644
--- a/tests/belts.scad
+++ b/tests/belts.scad
@@ -67,7 +67,7 @@ module belt_test() {
     translate([-25, 0])
         layout([for(b = belts) belt_width(b)], 10)
             rotate([0, 90, 0])
-                belt(belts[$i], [[0, 0, 20], [0, 1, 20]]);
+                belt(belts[$i], [[0, 0, 20], [0, 1, 20]], belt_colour = $i%2==0 ? grey90 : grey20, tooth_colour = $i%2==0 ? grey70 : grey50);
 }
 
 if($preview)
diff --git a/vitamins/belt.scad b/vitamins/belt.scad
index 48b9da5..7c9d51f 100644
--- a/vitamins/belt.scad
+++ b/vitamins/belt.scad
@@ -41,9 +41,7 @@ function no_point(str) = chr([for(c = str) if(c == ".") ord("p") else ord(c)]);
 // We model the belt path at the pitch radius of the pulleys and the pitch line of the belt to get an accurate length.
 // The belt is then drawn by offseting each side from the pitch line.
 //
-module belt(type, points, gap = 0, gap_pt = undef) { //! Draw a belt path given a set of points and pitch radii where the pulleys are. Closed loop unless a gap is specified
-    belt_colour = grey20;
-    tooth_colour = grey50;
+module belt(type, points, gap = 0, gap_pt = undef, belt_colour = grey20, tooth_colour = grey50) { //! Draw a belt path given a set of points and pitch radii where the pulleys are. Closed loop unless a gap is specified
     width = belt_width(type);
     pitch = belt_pitch(type);
     thickness = belt_thickness(type);