It's easier and probably better to do it with a paint editing program, but there is a way to do it by making a new shader for the texture. Open pak0.pak and look for the shader of the texturefolder you are using and extract it from the pak (eg. misc_outside.shader) Then look for the texture and copy its part into a new shaderfile you make with notepad. Save as 'yourname.shader' and put it in main/scripts folder.
For example let's assume the following is the shader, be sure to edit the parts in bold text:
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the values for rgbGen constant can be from 0 to 1, the lower the values the darker it will become, 0.30 0.32 0.20 would be nice I think. This way you can also change the colour of the texture. You won't see the effect in Radiant, but only in game. There's another trick to easily play with the values to get it right if the colours are not right yet. After you load up your map, go to the console and type: editshader yourshadername. Now you can change the values and save them(ctrl+s). Type vid_restart to see the change.
@Blue60007:
Well the way I described doesn't overwrite the original shader as you make a new one;
textures/yourshadername/yournewtexname. With a new shader and texturename you can borrow a standard texture. You can savely take one texture and use it for different purposes by writing another shader for it, as long as you change the name and put it in a separate shaderfile.
If you'd go and edit the texture yourself you'd need to make a new shader anyhow. But I believe Atomic wondered if it could be done in Radiant without editing the texture itself and this is just a way to do it. Nonetheless I too would prefer manually editing the texture in photoshop as the result is much better and you have lots more freedom.