Fix typo

lib/compare.js

@@ -42,7 +42,7 @@ module.exports = exports = function (proto) {
           throw new TypeError('The path for the diff output is invalid');
         }
          // graphicsmagick defaults to red
-        if (options.highlightColour) {
+        if (options.highlightColor) {
             args.push('-highlight-color');
             args.push(options.highlightColor);
         }