This tutorial will show you how to get rid of the blue pencil marks underneath a lineart. I usually draw with a blue pencil and lineart it with ink so that I don't hassle with transfering it on different paper or tracing etc. I think this is in reference to the Disney technique (?) or something. The next part will be to seperate the lineart from the white, so we have a transparent lineart where the color layers can go underneath without ever having to set it with Multiply. This is also helpful if you want your lineart in a different color instead of black. I hope you find this helpful in some way.
If you don't have a blue pencil and also prefer to keep your intial sketches or mess up with inking, you can scan your sketch onto the computer and convert the line into a light blue (you have to adjust it to a certain blue for it to work properly) using Image --> Adjustment --> Hue/Saturation with Colorize check marked. And then print that out on a smooth paper preferably and ink it with pen. That way you have your original sketch and the lineart. Have fun.