Had some people ask me over the last couple of years how I colour on Photoshop, so here goes.
1) Scan in at 1200dpi
2) Open in PS and use Image > Adjustments > Threshold tool
3) Reduce image size to 300dpi
4) Go to Channels tab and make a copy of the "Blue" channel
5) Go to Image > Adjustments > Invert
6) Use the "marching ants" tool to select the inks
7) Create a new layer
8) Paint bucket black into the selected area
9) Clear the background layer
10) Create a layer between the background and the inks to colour on
This is my tried and tested method, and exactly what I did for the O'Messy Life cover. I use a Wacom tablet and a whole bunch of Polygon lasso tool when colouring on that intermediate layer and it's a much messier business than it appears when you remove the inks layer from above:
In many ways, however, it actually looks much softer and easier on the eye than when my clumsy line work is slapped over it. Never the less, this is the finished product! More when I start work on the back cover.