Hey there primitive.
Give me a few minutes and I'll make a small tutorial-like post to show you how I pick colors. By the way, the tree seems quite decent, much better than the apple/cherry art. You just keep practicing and you'll keep improving!
(I'll edit this post with the info!)
--- EDIT ---
Take these example sprites. One is a small bush and the other is a little rock.
Look below the sprites, you'll see the color ramps I used. I didn't only use lighter shades of the same hue, I did something called
Hue Shifting, which is a fancy name for moving across the chromatic spectrum to pick adjacent colors. In the bush, darker shades tend to be cooler/bluish, while lighter shades are trending towards yellow green/yellow.
The rock is similar, from a darker, more reddish gray they move toward a light pink for the highlights.
Also another tip, try to experiment a lot with color palettes, look at this art example. I first created the lineart and then the color blocks in a very light pastel palette. But I experimented a lot with hue variations, contrast variations, etc. This allows you to have a wider selection of possible palettes.
Hope this helps!
K