Default color: White

Shared/css/main.less

@@ -32,6 +32,9 @@ canvas {
   position: absolute;
   top: 0;
   left: 0;
+  &:first-of-type {
+    background: white;
+  }
 }
 
 .separator {

Shared/js/shared.js

@@ -9,6 +9,7 @@ $(document).ready(function() {
     fill: false,
     lineWidth : 2,
     color : 'black',
+    bg: 'white',
     type: 'sketch',
     lineCap: 'round',
     lineJoin: 'round',