Incorrect colored blocks in noVNC (Sigmatek PLC)

When connecting to a Sigmatek PLC, the frames often have incorrect colored blocks in background areas. This is the image as displayed with TightVNC:
![image](https://cloud.githubusercontent.com/assets/6048186/4935329/2b29a4a6-65ab-11e4-8ea8-3159d4efce32.png)

In noVNC it looks like this:
![image](https://cloud.githubusercontent.com/assets/6048186/4935337/46b865cc-65ab-11e4-905f-fea945751be2.png)