In Spring or Summer, look for subtle modualtions of green among the trees. This can often be seen better in overcast lighting rather than under the unifying influence of bright sunlight. Freshly budding trees often have a warmer green hue while trees that have spouted leaves a while ago show more clorophyll in a display of cool green colors. Different species also show shade differences and can be set apart by these tonal differences further. Don't be afraid to exaggerate these difference to find the rthymn and poetry in a scene. Finally notice and include any atmospheric haze in your landscape by adding water to your pigments to lighten colors used on elements that sit deeper into the scene. Atmoshpheric haze will reduce contrast and lighten the values overall. In late summer haze, the distant trees and buildings will often look like ghosts.