I'm deeply impressed.
I do have a question: How do you choose which colors to use? I've seen in paintings where while at first it may seem the artist only used one color, they use like 5-7 different colors. I've never been able to do that ;_;
One of the things to keep in mind when picking colours is whether you're going for a warm palette or cool palette. Most colours have a warm and cool version of itself and it's best not to mix cool colours with warm colours. eg: If you take yellow and add a tiny bit of red, it'll become warm. If you take yellow and add a tiny bit of blue it'll become cool.
So for this particular piece I stuck with warm colours. The orange hues have more red than yellow while the green hues have more yellow than blue.
Also keep in mind that in real life things are never just one colour. Something that is primarily red, for example, will probably be reflecting colours from other objects. If you observe and apple in a tree, you'll notice that there might be bits of blue reflected from the sky and bits of green reflected from the leaves.
Those are just some basic things that I keep in mind when picking colours.