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Just a composition nitpick but I feel like there should be some foliage or other foreground element in bottom rightish/rightish area. You've got that tree rather near us on the left and it seems like it would balance things out a bit to chuck something in on the right.

As for color I recommend starting with a more neutral base color so you avoid getting too saturated too fast. Keep in mind the most saturated areas are usually those most directly hit by light. Anything in the background or distance should be less vibrant (more towards gray than 100% saturation) since it gives the illusion of depth and prevents the background from distracting us from the foreground or subject.

It'll have to get a hint more towards finished before I can give much other critique.

I'd like to make sure you understand i'm meaning vibrant, saturated color. So like when you get the paint directly from the tube, it's 100% saturation until you mix it with white, black, or a complimentary color. The opposite of a completely vibrant color is gray.
So when you're picking your colors in (photoshop? Painter?):

Anything in that circled area is going to have very full saturation, whereas the stuff to the left is more neutral.

This is completely optional, but try using the color dropper tool on an actual photograph of trees that look about how you want yours to (as far as color, light) I did a google search and found this one. That's really damn green, right? Really vibrant? Just choose the most bright, most vibrant green areas and then open the color pallet and look at where they are. Chances are even some of the most vibrant places will be more towards the middle than you might think.

So what i'm trying to encourage you to do is avoid the 100% saturation, because at the moment all of your colors are very vibrant and it is going to knock down the impact of the image.
If you look at this image the majority is in shadow and of a fairly neutral vibrancy, especially in the background. That patch of grass where the light is hitting, however, seems so much more vibrant because of it. So you get more impact when you use a large range of vibrancy and values (value means black to white, it's what you think of when you think shading).

As for not painting every individual tree, why not make a block of the leafy color up top and one block of the trunk color below and then sort of scribble in pseudo details? Of course you'd want anything in the background to be lighter in value (closer to white) and less vibrant (closer to gray) than anything in the foreground. It creates an illusion of distance.

I don't wanna be a s**t, but that thing I suggested about eyedropping the color from photos? Try it on a few of those artists. The second is indeed working in very vibrant colors and relying primarily on value to distinguish things, but both Loish and the first gal are working heavily in neutral tones with just splashes of vibrant color.

I'm not saying you can't use vibrant colors, i'm just saying to represent a space like the one you are painting, things that are further away will indeed look further away if you choose lighter, less vibrant colors, and they will look very close up and distract the eye from your subject if you leave them at full vibrancy.

And yeah I just learned about color theory last year in college. Blew my damn mind right out my skull. Cool stuff biggrin

I'm just nitpicking on something I noticed you doing that could be pretty easily modified, if you don't follow my advice it's no skin off my nose. I do think you oughta at least try it out though and just see what it looks like.