Don't be afraid to color, it's how you get better at doing so.
You can trace it and color that one, so the anxiety of ruining the original disappears.
No light box needed, just press it against a window if you have to.
Keep drawing, fan art is a good way to start and it's smart of you to pick characters with drastically different body types to help keep a variant in your artwork.
As for the line work, I would try looking into varying your line width, I think it will help your artwork not to look so flat.
Here are some tutorials on how to do so:
X X X
Speaking of things being flat, your first character could look much better if you added shading or even simple shadows. Even just three layers, a highlight color, a base color and a shadow color, could make your drawing pop way more significantly.