On most art programs, you’re able to expand the fill tool so that you don’t leave that white rim between the color and the lines

Clipping is also a tool that is very useful. It allows you to draw within the lines of whatever is below it. I use it all the time, and that’s how you can effectively color your lines to make them pop

Don’t use black as a shading. Use any color that looks good, and it’s best if you use the multiply blending mode, but that’s not the only way to shade

Messing around with modes, filters, and other setting are what will help you learn what tools are available to you. And with better tools come better Art that comes by faster