Improve consistency of validation error handling

Utilises common path of existing invalidParameterError

lib/colour.js

@@ -63,7 +63,7 @@ function grayscale (grayscale) {
  */
 function toColourspace (colourspace) {
   if (!is.string(colourspace)) {
-    throw new Error('Invalid output colourspace ' + colourspace);
+    throw is.invalidParameterError('colourspace', 'string', colourspace);
   }
   this.options.colourspace = colourspace;
   return this;