Thank you! :D First of all, check my faq, there are some tips. Anyway… I pick colours randomly, I just pick what I think will look good. If I’m not satisfied with the colours I chose I change them adjusting hue, saturation, brightness etc. I often blend colours on the canvas. Colour layers are also very helpful :) as well as colour balance options I guess. I don’t have palletes, it’s more intuitive to me.
Here’s how my colours for that Ron drawing looked like at the beginning:

They were darker and colder but I changed it adding colour layers.

I hope it’s not too messy lol, I was trying to show some important things about colours and how I do stuff xD I highly recommend learning about colour theory! (there are many great tutorials and books you can learn from) Even if my drawings aren’t very realistic knowing about colours and light, how they work together and what they do to each other is extremely important!
Also, draw from life, draw from references, experiment.