Photoshop has a number of artist-friendly options for making "realistic" trees and leaves. The "Scatter" and "Shape Dynamics" brush modifiers create pixel scattering which looks like leaves. The "Brush Creation" option allows you to make a "digital rubber stamp" in exactly the leaf shape you want, and then the aforementioned options will scatter your leafy image wherever you want to place it. I traced real leaves picked from the big pile beneath my parents' maple tree. As for color, Photoshop has the "Color Dynamics" option which makes your brush appear to shift colors while you are applying your virtual paint. All this is a fair imitation of the traditional painter's use of mushy brush ends dipped in a dry mix of paint and then dabbed onto the surface. In this way, nice seasonal images can be made that look like a calendar scene for October or November.
Here is what a real scene of calendaric Fall looks like. I took this at the Nashoba Valley Winery where I gave my mother a wine tasting for her 89th birthday. "Real," of course, must be qualified in photography. The colors of this photo have been Photoshop enhanced so that they look the way fall leaves really look to our imagination.