Hi there, I have a picture with a light wooden structure in it. When I raise the contrast to see this structure more clearly, the color differences in the wood also become more obvious. That's what I want to avoid. Should I just copy the layer, desaturate it on the other layer and use this to increase and lower the light values in the picture? Does that make sense? How should I do that? I'm not yet an expert on Photoshop 7. Help is very much appreciated. Thanks.

I've done some things in which I have made an "overexposed" copy and an "underexposed" copy of a photo to increase the contrast, whiten whites, gloss up hair, etc. Use a layer mask to reveal the layer you want visible -- original, overexposed (lightens), underexposed (darkens, increases contrast).

Hi Tint, welcome to the forum :)
Adjusting the Levels or Curves should not be changing the colour, just the tonality. If there is an apparent colour change, it means that the colour is actually there and the tonality change is just bringing it out. Compensate with a Hue/Saturation adjustment layer after Curves or Levels

This is a case where "adjustment layers" come in real handy ;)

:)
retrocity

Thanks for your hints. I'll just try these suggestions. ;-)