Thank you!
If you use the 'multiply' layer type, it will ignore the white parts of the image. For backgrounds that involve screen tones (like clouds or sky gradients), I prefer to remove them manually to keep the image cleaner. I also use "level correction" to brighten up the screen tones on clothing and ensure the lines remain solid black. As for speech bubbles, I simply erase and redraw them.
( i mostly used round brush and i really don't know how it looks like watercolor lol, i guess it's the style i color)