Muddy colors? Yeah I get what you are saying.Are you using multiple layers? Make a layer for your shadows and a layer for lights. A layer for special effects / highlights, spectaculars. I still struggle with muddy colors, was painting a landscape and took a step back and I'm like "Ugh this is all gray and muddy right now". Just the process gotta push through that and complete the painting. It's almost 4 am and this is a lot of stuff to explain in a post. Google figure drawing basics, go on pinterest etc. for reference, practice making people. Get a stack of paper and do that? if you can get a little note book try drawing people irl. The more you do it the more effortless it becomes. That's it really. For fundamentals in just basic drawing practice warm ups involving S, circles and straight lines, diagonals. If you can bust those out smoothly you can make bold confident lines when you want in your drawings/ paintings.