Green trees, huh? Sounds easy... all you have to do is slap on some green paint and you're there. You look at the forest you're painting. The only colors you see are green and brown. So you paint the painting green and brown and you end up with a muddy, blotchy mess that has no depth and looks nothing like a forest scene.
First mistake - you may only see green and brown, but there have got to be some other colors in there - red, blue, black are safe bets - and trees generally have yellowish highlights somewhere. Tree trunks are often more gray or purple than brown. So paint these colors, not just greens.
Using more than 3 colors in a wash can get muddy fast!