Monitors use a three gun approach to display colors on the screen: ( I think this is true for USA, in Europe it is different ), but, it uses a gun for red, green, and blue - hence the RGB term, and the connector named for your monitors cable connection... Possibly, one of the guns is getting old? Do you have a way to "degauss" the screen?

If you think your monitor is OK, or it is of the newer purchase, it may just be the "theme" you have on your screen for Knoppix... You can change your themes through the following: KMenu -> Settings -> Control Center -> Appearance & Themes ( either you can change the background, or the Style, or the Window Decorations, any or all )

Okay, I've tried the degauss button and nothing different. Monitor is 2 years old, pretty new I guess and olny have to try to change the theme.