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Hi there! I thought I might stop by and give you a helpful critique!

It's my guess this is digital art done in Photoshop, I don't personally use Photoshop for lineart, because I've found the lines are rather unsteady when it comes to sketching- however I know a great deal of people who do use it, and most likely have a steadier hand than mine. But, I've noticed your lines look almost binary and don't have a real thickness to them. If you're looking to do lineart [much like manga and comic book artists do] I would suggest using a hard brush for your lines, and a soft [and whatever other brushes you have- you mentioned 'watercolor'] brush for painting/coloring. Also, actually color the colors in- rather than using the bucket tool, because when you do that, there's always this white line between the colors and the linework that makes a picture look messy and unfinished.

Also, your shading is rather inaccurate- you only darkened the colors around the edges of the lines- but that's not how shading works! I would suggest studying value for help on how light and shadow works. [I struggle with this as well.]

CtrlPaint has a lot of information on the basics and beginnings of digitally painting! It's composed of short videos, and I think would be very beneficial to your growth as an artist!

You also said you typically work in black and white- practicing in color is good, but if you decide to study values, practicing in greyscale is easier. Actually, Kienan Lafferty has a tutorial about taking pictures drawn in greyscale to color here that could also really help! [I recommend watching more of his videos, as they're all very helpful]

Hopefully those links help! Good luck on your artistic journey! :>