B2Q: Comics Colour Tutorial, Pt. 1

=== 2013.12.08: While there may still be some academic value to this tutorial, and while its principles remain sound, I am quite sure there will not be a concluding part written and so it may behove you to look elsewhere for a more complete tutorial for simulating the look of early comic book printing. --L.

    I suppose it's ironic that we'd even want to make our art look like it came out of an old comic book when, at the time they were stuck with it, artists lamented the incredibly poor reproduction quality that was the industry standard for decades. For a lot of us, though, that look instantly recalls to our minds our childhoods, and the excitement and engagement we had with our badly-printed heroes and villains and their off-register escapades through space and time. I guess for you younger folks who were born after digital colouring started in the early '90s, you're drawn to something else about it, but we're not here to investigate the feelings it evokes, we're here to capture that look and use it to evoke those feelings in others (and ourselves).
    This tutorial is necessarily GIMP-based, due the fact that I don't use Adobe Photoshop. But, a massive and expensive program like Photoshop should certainly be able to do all the same things that GIMP does, so the principles herein are applicable there, although you'll have to figure out what tools and terms your program of choice uses.
    Before diving into what to click, it's important to know why we're clicking it, so I shall now drive-away the less studious amongst you with a couple paragraphs of principle; those of you that persevere will arrive at the first bit of practice.
    I will assume the reader (that's you) is already familiar with the four-colour printing process, and how colours are represented with it by overlaying dots of cyan, magenta, yellow, and black ink. Prior to printing an image, a process called colour separation must be done to produce the four individual printing plates for each colour. For full-colour artwork like paintings and photographs, colour separation is done either by the use of cameras and filters or by computer software, and produces four colour plates with smooth, almost imperceptible gradations in tone, and these plates when printed together give to the eye the impression of all the colours of the rainbow, plus all the colours in-between. Comic books, on the other hand, for the majority of their history, were prepared for printing via manual colour preseparation. This is a process in which the colour plates for each color are made by hand. 
    More specifically, flat-tone (or flat-tint) preseparation was the order of the day. In this process, a plate is created for every flat tone of every colour that's being used. For example, a plate would be created for all the areas where solid yellow would print, then a plate for areas where a 50% tone of cyan would print, then a plate for 20% red, then one for solid red, then one for 40% red, and so-on, then all the tone plates of a single colour were screened and combined by the printer to produce the final plate for that colour.  More combinations of tones produce more colours, but as you can imagine, at a certain point it becomes both expensive and laborious to have some poor colour separator doing a bajillion plates per page. So, in the interest of keeping things cheap and easy, comic book publishers limited the number of tones a colourist could use to combinations of 100%, 50%, and 25% each of cyan, magenta, and yellow.
    This limitation results in one of the immediately recognizable elements of the old comic aesthetic: the limited palette. With just three tones of three inks, we end up with only 64 possible colours. There's a bit of lingo to learn, here, with regards to the names of those colours, and I'll let Todd Klein tell you about that: "Colors were defined by a code system. Y stood for Yellow, B for Blue [cyan], and R for Magenta [red], matching the three colors of printing ink other than black. ... B represented solid or 100% blue ink, B3 was 50% blue ink, B2 was 25% blue ink" [good article to read]. With those codes, colours can be referred to by formulae: BR2 would be 100% blue plus 25% red, Y2R2B3 would be 25% yellow plus 25% red plus 50% blue, and so-on.
    The change in terminology, here, from cyan to blue and magenta to red, is, in a way, significant for what we're dealing with when we try to colour art on the computer and get it to look like print. Magenta, from an RGB colour-model point-of-view, is a blend of 100% red light and 100% blue light. The colour this produces on our screens is quite different from the colour produced on the printed page. If you were to dive into colouring a piece right now, armed with the incomplete knowledge that you have only to make sure that you use 100%, 50%, and 25% blends of cyan, magenta, and yellow, you'd end up with something like Figure 1. The colors are searing. Instead of getting the look of Action Comics circa 1964, you get the look of Microsoft Paintbrush on Windows 3.1. Printers' magenta is not RGB-magenta, and printers' cyan is not RGB-cyan. To complicate matters further, red/cyan for one comic book is not the same ink formula as red/cyan for another comic book.

Fig. 1  -  Just using the RGB equivalents of cyan, magenta, and yellow produces a frightfully intense palette.

    So, our first order of business in getting our comic-accurate colours is creating our own palette of 64 colours that actually look like the 64 colours we'd see on the page. Are you ready to get interactive? 'Cause this is where you start mixing your own inks.
    If you download and open BRY_chart.psd, you'll find a four-layer image of an 8x8 grid. The three layers, B, R, and Y, are going to contain colour swatches of your inks and their 25% and 50% tones. The layers are set to the "multiply" blend-mode, so once you've selected your ink colours, all the in-between colours will be made for you. To get going on this, turn off the Y and R layers, then make B the active layer. Next, go to Colors->Colorize (or Alt+c,z). The Colorize tool defaults to hue:180 sat:50 and light:0, and this leaves us with two bands of blue and one of black. The black areas are what we want to dial-in (be sure to switch-on 'preview') for our blue ink.

Fig. 2 - BRY_chart.psd open in GIMP. The (B)lue layer is active, and the Colorize tool has been launched.

    If you change the values to hue:180 sat:100 light:50, this sets our blue ink to RGB-cyan. But we don't want that, because it looks totally wrong. The blue in Figure 3, from a 1962 issue of Strange Adventures is much closer to 190,50,50. But for my tastes, I'm gonna dial-up the saturation to 75, and the lightness to 55. This gives me a blue with a bit more punch, while still being being roughly the same hue as in the comic. It's more like how the ink would appear on a cover, actually, where the paper stock was whiter than newsprint, and absorbed less ink, which is what causes the desaturation on the interior pages.

Fig. 3 - Getting away from RGB-cyan, use a comic as a reference to find your ink colour, then make whatever adjustments suit your taste.

    Once you've picked your blue of choice, hit Ok, and repeat the process on the Y and R layers. When you've got your ink colours picked, turn on all the layers and you'll have a beautiful 64-colour palette with which to colour your masterpiece. For now, you can flatten the image and use the eye-dropper tool to sample colours and then paint with them in another window, but in the second half of this tutorial, you'll be creating two palettes and working in indexed-colour mode (8-bits of power!) and from there get into half-tones (I know, I know, everybody just wants the dots right away, but you can't have the dots until you know what you're doing with your colours) and ways to make things look authentically old-and-crappy.

Fig. 4 - Even though this is still a pristine digital image, it's starting to look "right". Getting the colours is of chief import -- halftone dots and paper texture are just gravy.