Hey, I apologise if this has been asked before. But got any tips on how to make that sort of lineless coloring style look neat? It's so cool looking but I can't seem to manage to make it look nice.

Without outlines, it’s really important to have a sense of which colors and tones compliment and contrast each other, so nothing bleeds together. 

Best example? Jack.

Knowing your tones is really important, so even if the environment is the same color as the characters, they can remain distinct.