This is how I do colour flatting for CorpseTalk. Each week I think “maybe this would be useful for somebody” so I’ve finally gotten around to writing it up. Colour flatting is basically just filling all the areas of your drawing with randomly generated colours, which you can then go back and easily fill with the actual colours you want.
We’re going to be using the excellent (and free) BPelt plugins, so the first thing you need to do is go to their site and install them, if you haven’t already. BPelt does most of the hard work for you, but there are quite a few steps that you need to go through to prep the files to get the best results. That’s where you need my Photoshop Actions! Right click, download and open with Photoshop to install them.
I scan my line art at 1200dpi, B&W (no greyscale). It looks like this: 
Then I shrink it down so it fits on the page. I like to think this is giving me the tightest possible linework. Maybe I’m just being superstitious… Anyway, it means I end up with this 
Notice that this file now has anti-aliased edges. Not ideal, but my computer can’t handle 1200 dpi colour files.
I copy and past this is a new file [CTRL A > CTRL C > CTRL N> Enter > CTRL V] Then run MULTIFILL STEP 1 from the bpelt_multifill_helpers actions.
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First it resizes the image to 300dpi – on my computer, the BPelt Flatten plugin can’t handle larger files. Then it converts the file to B&W – BPelt doesn’t like anti-aliased edges, so this removes all the anti-aliasing. Then it converts back to CMYK. It copies the current line art layer (we’ll need it in a moment) Then it runs the BPelt multifill plugin – this assigns random colours to all the areas of your lineart. Note: you may need to play around with your settings depending on your drawing style. These are mine:  Then it runs the BPelt Flatten plugin, this removes all the black lines and expands the areas of colour so they meet up. Then it pastes the line art layer on top of the colour layer (from when we copied it before), selects the new line art layer and chooses the pencil tool.
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Now you have your line art with random colours for all the areas. However, if you look closely you’ll see that there are lots of areas that are joined with one colour when they should be two, because your line art is full of gaps (at least mine is). Also, there will be many areas where there are two areas with separate colours which should be connected, but you drew a line between them. So now go around the line art layer and wherever you see a line that should be connected, connect it using a black pencil tool. 
And wherever you see a line separating two areas that could be connected, break it using white pencil (the line art layer is set to multiply). 
Does that make sense? Essentially, you are fixing all the bits of your lineart that BPelt can’t handle. Don’t worry about making a mess here – all of this is just to get the best colour areas you can, we’ll be going back to the original lineart shortly. Make sure you have pure black and white selected as your foreground and background colours. Remember that you can use X to toggle the background and foreground colours. When you’ve done this for the whole page, run MULTIFILL STEP 2
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Pretty straightforward – it just runs Bpelt multifill and flatten again, using your new, edited lineart, and then resizes the image back up to its full size (remember we shrunk it previously). Note this is done using “nearest neighbour” so it keeps the jaggy aliased edges – very important.
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If you still aren’t happy with the colour flatting, you can press CRTL-Z about 5 times, until the 3rd layer disappears, and do the previous step again, but it’s probably easier to just fix it at the next stage.
Now you have your colour flats done. Note the edges are quite jaggy, but that doesn’t matter too much as they’re going to be hidden. Copy the colour flats back into your original file [CTRL A > CTRL C> go to your original file > CTRLV].
Line up the colour layer with the lines – I reduce the lines opacity so I can see through them, zoom in and then move the colour layer so the lines hide the jaggy edges.
And that’s it! Go around with the paintpot tool (tolerance 1, no antialisaing, contiguous, this layer only) and fill in all the areas with the correct colours.
One more trick – I’ve assigned the other action in that set that you downloaded, Expand 4px, to the F2 button. So any time you find an area that hasn’t been correctly filled, for whatever reason, you can go Wand tool [W] > select the area or areas > F2. This will expand the selection so it’s underneath the black lines, avoiding any antialiasing jaggies. Note you need the wand set to no antialisaing, contiguous, all layers.
And there you go! I hope this will speed up your workflow and free up time to make more awesome comics. If you have any further suggestions for improving this system, or this post for that matter, please share in the comments. I’m always looking for ways to make it better.
