OK i've made this colouring the background of tech tree:
I found the colour to make black (but again researched techs wasn't distinguable)
I played a little bit and changed tech_panel01.tga
to this
mix of orange and gray (probably i could desaturate and darken it a little bit so the effect could be even better)
And here's effect (it isn't black, but dark brown fits better to new gui than blue)
http://img529.imageshack.us/img529/2408/techtreefixedfd8.jpg
To find good filter color of this .tga (if you don't have knowledge about colour mixing) i recommend first:
1.make new image and with paint bucket fill it to blue
2. set transparency for example to 50%
3. add new layer behind the blue one
4. experiment with different collours unless you get something similliar to the effect you want
5. Color tech_panel01.tga and check in game,
6. Adjust colors in tech_panel01.tga (saturate, desaturate, change hue, lightnes, darkness) if needed