First, find out the reasons a monitor would make a high pitched noise, then work backwards to find out why this happens in your colored output scenario.

I think it might have something to do with capacitors, I had problems with my video card or mobo making high pitched sounds and I think I remember reading that the capacitors were making the sound. It only happened when entered a game or app that made heavy use of 3d rendering.

So my guess is the app that outputs in color is making your monitor or video card enter a state that uses a lot of power, or otherwise for some reason causes the high pitched sound.

I could be totally wrong though.