I agree that the colors of the default tree shaders are way over-saturated and bright. Bryce is the same way. They end up looking like toy train set (not model railroad) trees.
A good starting point for more realistic trees would be to grab Patrick210's tree shader from either digitalpainters.net, or from polyloop,net  It's a freebie layered shader and makes good use of translucency for realistic backlighting. You could also grab HowieFarkes's terrific free snow scene from the Carrara freebie forum at DAZ. It includes his great snow-covered trees and is quite instructive if you analyze the plant editor and material settings he used to achieve his effects.