It’s pretty easy. Just head on to Image >> Adjustments >> Curves
Click on the Options, select either of the first three selections. Then change the Shadows to any color you like. I’m used to blue so I do that.
Now when you use Auto-tone, it automatically uses that setting.
Then just create another layer and draw the clean lineart there (or just draw directly on top of the sketch if you hate yourself lmao)
Say you accidentally drew your clean lineart over your sketch lineart (like me!)
Go to Image >> Adjustments >> Black & White and this little window will appear
Bump the Cyan all the way up or until the sketch layer disappears.
TADA!
Hmmm but now the lineart isn’t transparent like I wanted it to be….. what do?
If the image is flattened, unlock the lineart layer first (assuming the lineart layer is the background layer)
Go to the Channels tab and click on the little dotted circle on the bottom
This will happen:
Congratulations! You have selected all the white stuff surrounding your lineart. You can either inverse select to get a selection of the lineart or just remove the white stuff by pressing Delete.