What software are you using? I'm using Photoshop, but I think GIMP would be the same. Manipulating the values will still you give you obvious differences of color when tiled, since the dark spots also have a different hue (as well as brightness value) from the other parts of the image.

Thanks Prinz Eugn - that's exactly what I was looking for. I did try it myself, but I couldn't get anything reasonable. I converted it to graysale in GIMP and then messed around with the curves and levels for a bit - I couldn't get an even colour. Would you mind telling me what tools you used? Thanks!

What software are you using? I'm using Photoshop, but I think GIMP would be the same. Manipulating the values will still you give you obvious differences of color when tiled, since the dark spots also have a different hue (as well as brightness value) from the other parts of the image.

I'm using GIMP in this case, because Photoshop Elements 8 doesn't have a curves editor unfortunately. It's one of the few reasons I bother to even install GIMP.

When the curve editor is open, I use the color sampler / needle tool to click on the dark spot in the image, and that shows me where it is on the curve. Then I just move that point up or down until I even everything out.

There are many ways to go about this.

In the interest of more options (using photoshop):

Put the image on the base layer.

Duplicate it and blur it heavily.

Set the blurred copy to the colour blend mode.

Yeah, high-pass filter does exactly what OP is looking for -- preserving high frequency details while removing low-frequency color variations. Here is how it looks if you combine the high-pass details with the average color of the original texture:

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