I don't know what is the problem behind the colors.

It could be the color depth, set in the screen section using a line like this:
And for individual resolutions in the Display subsections using:
Not sure why puppy linux xorg.conf had a depth of only 16 bits. Having less bits uses less graphics memory, which can sometimes allow the card to support high resolution screens (and other features). If you can get high enough resolution with 24 bit color, then it is a better option, though the change would primarily affect the graduation of color in images, rather than have gross impact on the color appearance.

Another possibility is that it is caused by one of the Intel driver options. These can be found in the xorg documentation. I don't understand these options well enough to advise you; the only two that I think would affect color are the ColorKey one and the Dac6Bit one.

It could also just be the monitor settings (as you have already tried). These aren't easy to get right; sometimes people make use of a modeline file (especially when the automatic recognition of the monitor parameters does not work correctly).

:( no difference by changing the color depth...

I noticed something strange: the system does an autologon and immediately starts gqview in fullscreen mode. The first image doesn't fill the screen completely so it has a black frame which is added by gqview.
The next ones are all 1440x900 dpi images and it's from there on in the presentation that the screen switches to darker colors.
If I exit the fullscreen loop, gqview keeps on switching images in it's window and the colors - on those smaller, 'preview' images as well the toolbars, background - are fine.
By moving the gqview windows to the left side of the screen, all colors get darker again including the colors of that window. This is the same for another window as well.

I guess it must have something to do with display settings but the question now is what/where...