I've noticed that in most texture mods, the mod makes the texture darker. Like for rocks, the vanilla rocks are low res but light grey, whereas all replacement textures are high res, but a lot darker. It changes the tone of the landscape quite a bit, which is why I mostly never use high resolution texture packs.

Has anyone else found this?

It depends on the texture pack. I use AOF Detailed Mountains for that very reason because it looks closer to vanilla.

But there's also the high resolution texture pack from Bethesda itself, why not to use it instead of texture mods?