Colorizing was done by –
A. Scanning the original art into Photoshop
B. Duplicating the original art a couple of times onto a couple of different layers. One layer would stay untouched in case I needed an undamaged version of the original image.
C. The original scan is duplicated again and then colorized.
D. The colorized layer is duplicated and then given a gaussian blur.
E. The three active layers (original, colorized and blurred) and then set to multiply.
F. I then erased out the parts of the original layer where I only wanted color to appear in the image (rather than color with a grey undertone).
G. I adjusted the contrast of what’s left of the original layer so that I get solid blacks and the greys are lighter.
H. I added another layer set to multiply and then added color for skin and the background.
I. On additional layers I added highlights or undertones of color.

I’m fairly happy with how it turned out. I’m especially happy with how little time I spent in Photoshop.