You have four colors in the trunk, which seems like enough to me. Your foliage only has two (well there is that third, but you only use it in once little place). I would add another color or two to the foliage before you add more to the trunk.
More important than the number of colors is how you use them. I see a lot more detail in the trunk than in the foliage. The foliage needs to catch up a bit. Get some more detail in there.