A quick and easy way to get rid of blue lines in Photoshop. This is useful as well if you want to remove lines from folio pad paper after scanning.
For other colours, just do the same but adjust the relevant slider - red for red, green for green, etc.
This method will give you a flat image on white.
To get a transparent image with just your linework, use this awesome tutorial by ~JollyRotten
PS, thank you to =Ben-G-Geldenhuys
for teaching this to me <3