Very cool art! There are a few things you can do to get colours that go together which involve a little bit of colour theory which there are lots of lessons on. I do have a couple tricks that can work pretty quickly though. The first is to pick 2 complimentary colours and just change desaturate and change the values until you get something nice. The second is the way I decided to go about it for this piece. I actually misunderstood the question and thought it was a value issue so my process to changing the colours became very messy. So there is a thing called colour triads. So by choosing colours from the colour wheel that are 3 spaces away from each other gives you a triangle. As long as you manage the saturation and value of each, they tend to go together very well. You can have a lot of one colour and a little bit of the others, or try to spread them equally. It all depends on the feeling you're trying to get. I made a video of the process and the 2 other palettes I went with. It may or may not be helpful. (https://youtu.be/j7X9Unfoxe0)

Not a color expert either, though I thought you had a few too many colors going on and from what I do know you usually want a dominant color, and with the size of the cloak I thought red would be the choice here, so I shifted the browns of the shirt sleeves and boots, as well as the axe handle, and had them lean more red. The I also added green with the shoulder armor and earrings. Lastly, I know the dark green you had on the left behind the beard was a deeper shadow, yet I thought that was one green too many, so I swapped that with the shade of your other shadow green.