How to change screen contrast of my android device?

Question

Servus!

I am coding a demo for a prototype with a build in Z1 android phone watch. Problem is that the default contrast setting for its display is pretty bad. Depending on my brightness setting I can choose between black looks like grey or white looks like grey. So I went out and found an app called "Screen Adjuster" to change the screen contrast. But the thing is that this only let me lower the contract, but I need to raise it to get rid of the grey-color-problem.

Now I looked how to change the screen-settings on my own and did that at least for brightness. But now I got stuck searching for a way to get to some sort of contrast setting handle. I wonder if the app did that by some sort of filter overlay or something?? At least this would explain why its not possible to raise the contrast...

I need ideas!

edit: The device runs Android 2.2.1 and is already rooted.