(also applicable to getting coloured sketch lines off of a scanned image, and making scanned lineart transparent)
in the last few days i’ve seen like three people with this problem?? and it is such an easy mistake to make and annoying to correct if you don’t know the trick so here is a post starring this frog i found on youtube today about how to get rid of it fast + easy
ok so i’ve fucked this up really bad. worse than you have probably ever done. he is all on one layer with weird colours. DON’T PANIC EVERYTHING WILL BE OK
first you have to play with the saturation. make it as bright as possible; your eyes will hate you but it will be over soon. turning it up all the way tends to compromise your line quality a bit, which i don’t think would matter much if you work big and size down later, but harold’s poor cute face is noticably more pixellated and terrible when i turn it up to 100. probably best to do it manually with adjustments > hue/saturation and zoom in while you are adjusting to make sure it’s not eating your lines.
your make a new layer on top of it, fill it with PURE red or blue (green does not work for some reason) and set that layer to HUE. then you merge that layer with your lineart/sketch.
now he looks like this!! you have succeeded so far in making your drawing look like unicorn puke, and then you made it look slightly less sinful, but still bad. i have tricked you. there is no hope left
UNTIL YOU DO THIS. go to channels and click on the colour you picked. now all those coloured lines appear to be GONE. but we are not quite done yet. go to the bottom window and click this
and hit Ctrl + Shift + I to select all the visible lines in the image. THEN click RGB again (so that you aren’t working in shades of grey + blue/red anymore), switch back to the layer panel, and make another new layer. then fill that layer with black, or whatever colour you want your lineart to be. now deselect and delete/hide your sketch. you are now a wizard. YOU HAVE WON. HAROLD IS FREE FROM HIS TORMENTED LIFE ATTACHED TO THE BACKGROUND LAYER. HE CAN BE THE FROG HE WAS ALWAYS MEANT TO BE. CONGRATULATIONS.