First a disclaimer - This is not "How to colour on computer" It's "How *I* colour on computer". One thing I've learned from talking to others who have done it professionally is that although most of us use the same programs (come to that a lot of us seem to use the same machines) we all go about it in different ways. So there is obviously no right or wrong way to do it.
I colour on an Apple Mac mostly using Adobe Photoshop 3.0.5.(Version 4 should be out next month). My computer is getting quite old now but can still cope with comic pages at a reasonable speed (just). One thing you can obviously use as much of as you can get is RAM. My machine has 136 MB with 120MB partition for Photoshop. You can work with a lot less than that but you spend longer sitting around waiting for Photoshop to keep swapping things to and from the hard disk.
There is discussion elsewhere in this group about the resolution of scans used in comics. I work at a minimum of 300 ppi (pixels per inch). First as I use an A4 (8.5"x11") scanner I have the art supplied at print size. I scan this at 600 ppi in bitmap mode. A note here about threshold, this is the point at which any soft greyish edges to lines will turn to either black or white, it takes some experimentation to find out the right setting for your setup but once you've found it it shouldn't vary much. Some scan in greyscale and set the threshold in Photoshop but I prefer to do it while scanning. If the threshold is wrongly set the fine lines will break up, the tips of pointy lines will disappear or white out of black will fill in as will textures.
I then make a copy of the file and reduce the resolution to 300 ppi making a smaller file to work with (some publishers actually request the file to be supplied at 300 ppi which can work fine depending on the paper used) and can be combined with th 600 ppi line work later. I convert the bitmap file to greyscale and use the select colour range command with the tolerance set at 1 and the colour selected as black, this selects all the line work. I then cut and paste onto a new layer which gives you the effect of the black line work being suspended on a transparent layer above the background. I can then draw anywhere on the background layer and the marks will appear behind the black art.
Then I convert to RGB mode and colour in the page on the background layer in flat colours using the pencil and paintbucket tools. At this stage it looks like an old comic. I then use the magic wand tool to select each area of colour in turn and add shadow and highlight modelling with the airbrush tool or apply gradients or filters to background areas. Selecting the flat colours in this way effactively cuts a mask for the area as you would with adhesive film for a conventional airbrush.
(Note that when you select an area it is surrounded with a 'marching ants' marquee. This slows down screen redraw and causes a frustrating delay while airbrushing. You can get rid of this by using the command hide selection (command H) just remember that you've done it or you will find yourself trying to draw in the masked area and nothing will happen.)
If the file is going to stay at 300 ppi I then set the line work layer to preserve transparency. This means that you can draw any where on the layer but only the black will be affected, the transparent areas will remain transparent.If I'm going to combine with the 600 ppi art I would first disguard the black line work layer then using the image size command I would increase the resolution. After converting the 600 ppi line art to a layer as described earlier I would then copy to the file containing the colouring and set the layer to preserve transparency. For any effects I want to overlay the black like glows or smoke I would then create another layer once I'm happy with everything I flatten the layers and, if required, convert to CMYK mode.
(Note that if you don't want effects overlaying the black, something which is being chronically overdone anyway, you can just combine the 300 ppi colour work with the high res line work in a page layout program like Quark Xpress, the colour work does not need to be at anything like the resolution of the line work so this gives much smaller files.)
So there you have it, I just wish it wasn't so time consuming. It takes me a day to produce a page I'm happy with, I don't do silver or bronze standards (or zinc standard as Alan Davis described X-Men vs ClanDestine #1) so I don't think I'm ever going to be doing this full time but it can make a nice change of pace from inking.
I'm very interested in how others go about colouring on computer, if you don't want to go into it here feel free to e-mail me.
(Who hopes that this actually makes some kind of sense when you read it :)