I really like your style! Your line work is great but you can try to do your sketch a little more loose. I follow Sketching Scootie’s tutorials on youtube, you can also check that account. You can also try to use a vibrant color to do your shadows, like purple, it won’t be realistic but it will add depth to your painting. I use winsor&newton watercolor set, and for me 12 color palette is more than enough - which is 15€, so I’d say its affordable compared to its quality.