this is the easiest way to isolate
traditionally made lines in photoshop
open the scan of your drawing and go to
the channels tab if you don't see it
click window in the top bar and select
channels in that tab hold ctrl or
command and click on the one called rgb
this will select all of the white of the
page invert that selection with shift
ctrl i on a pc or shift command i on a
mac
back in the layers panel create a new
layer and add a mask make sure you're
marking on the layer not the mask and
fill your lines with color now you can
freely color under your lines on another
layer or even color your lines
separately if you need to there are some
ways to clean up any straight pencil
marks on your scan before isolating your
lines if you'd like to see a video on
that comment below