I just -- oh gosh, this is complicated to explain ahahah <3
I make a new Multiply layer above the pencil art and colour the areas that I DON't want to be transparent. Then you select the area around that coloured tracing you just did on the multiply layer, click back onto the pencil layer, and hit delete.
Then you can delete that multiply layer, cause now your pencil art has no white background! WOO! :3