>>2823675I sorta followed this guide, didn't really bother with the colored shadows part but everything else is pretty useful
Main point is that you need any drawing program that supports layer modes, which is almost every one these days, hell I used Gimp and it worked just fine
In my case, since I didn't have the clean lines and was stuck with having them on a (luckily) white background, I was able to put the line layer at the bottom and put the color layer on top set to multiply mode, so the white parts would not change the colors at all and the black lines would also not be changed by the coloring if they happen to overlap
I haven't tried it myself but I'm sure you could try messing with the color values of your original image if the background isn't pure white and go from there
