I want to emphasize Arne's point on contrast.
You might want to consider the fact that you cannot see most of your colors on certain monitors unless you are WAY zoomed in due to the darkness of the colors and how near they are to each other in terms of absolute brightness (this is the contrast-thing Arne mentioned!) so this lack of contrast not only makes your art very dull to look at (even if the construction of your tree and your chosen colors were spot on!), it also makes it painful to look at too because one has to squint and zoom just to see the colors, leading it to look like a flat paper cut-out (fly-swatted!) onto the background with only 2 colors visible at any other zoom level!
LIGHTING YOUR ENVIRONMENT
I want to also expand on the "lighting your environment" thing, as this can be an overwhelming concept to a beginner:
As Arne suggested, you should always consider your environment's lighting -- it's very important to indicate form, as well as for any mood, tone, or even any suspension of disbelief you want to convey! -- however, at the same time, you should also consider your own artistic framing as well.
For example, if the background trees are supposed to be subdued, depending on your pixel/rendering style and chosen level of detail -- 2 to 4 colors are PLENTY enough to render BG trees while also keeping them subdued since lighting actually /creates/ form through its contrast with shadow, so the weaker your contrast between light and shadow becomes, the more subdued (but potentially also FLAT as well, so be careful with this!) your subject will also become.
VARY CONTRAST WITH PROMINENCE
If you want a thing to stand out more in your environment, you'll want to give it much more contrast than anything else. The inverse is true if you want to subdue it. Vary the contrast between things in your scene to keep things in line with how prominent you need them to be at first glance to the viewer.
