I sort of noticed the same thing with screenshots.  Sweetfx is very cool, but I don't believe that is what we are seeing or necessary.
Suggestions:  
Option1_ Type .sliders in the game while clipboard is up.  This brings up additional controls for the plane skins (std shape), terrain, and trees.  They will be the bottom 3 sliders, and the only ones functioning in this manner in beta.  Lower numbers on the slider scale are darker.  Also, use the gamma slider under graphics details menu.
Option 2_ Use your graphics card's control panel and look for settings (sliders or curves) for colors, saturation, contrast, brightness, gamma, sharpness, etc.  
Option 3_ Use your monitor's control panel to adjust settings (same as above)
Lastly, I have found that a slight adjustment in hue from my video card has done a lot to "pop" colors more (I was surprised).