I've posted before about how full-color pictures are typically printed using four colors of ink: cyan, magenta, yellow, and black (CMYK). If you've got an inkjet printer, same deal. The first three colored inks are transparent, so that printing cyan atop yellow produces green, for example. Black ink is . . . oh, I don't know . . . not quite transparent but definitely not opaque. Semi-transparent? The key is that, although black covers better than the other inks, it doesn't cover totally. You can't just paint an area black and expect it to cover everything underneath.
I don't think I'm giving away anything about a book titled Whatever Happened to the World of Tomorrow by revealing that some of it deals with outer space. Now, on my computer monitor, when I color an area with 100% black, it looks as black as the inkiest ink puddle at the bottom of an ink mine on Planet Ink. But when I hand off those digital files to Designer Neil, he can push a button ("overprint preview" in Adobe InDesign) to approximate what they'll look like when reproduced with actual ink on paper. The result: something that appears solid black on screen comes out kind of charcoal gray in print.
Okay, I knew that. Learned it on Mom's Cancer. Black by itself doesn't cover as well as you'd like or expect. To help black ink look really black, you need to fortify it with other colors to make what's called a "rich black." It's like laying down a primer coat when painting a wall at home. As shown below, different color combinations yield different tints of black.
You might be tempted to mix all the colors with black--100% black plus 100% cyan plus 100% magenta plus 100% yellow--but that would be a gloppy mistake. You need to remember you're working with liquid inks on absorbent paper. Reality trumps theory. No one wants an ink slick.
What I didn't really have a handle on until Neil and I started talking was how those different tints of black would look together. For example, below is a panel with black and black on black: the shaded side of the Earth and the shaded side of the Moon on the blackness of space behind them. My goal was to make those three blacks very subtly different. To that end, the blackness of space has some cyan and magenta mixed into it, the blackness of Earth has a bit more cyan (hard to tell in this low-res image), while the blackness of the Moon has a greyish earthtone.
It took some effort to get the tones of those blacks to balance. If they're too different, the different elements can either wash out or become too prominent. For example, in my first try, the star background was too gray and the planets looked like stickers stuck on a piece of faded construction paper. I'm pretty happy with the look I achieved above.
Anyway, that's what I did today: went through the entire book looking at every big patch of black to decide if it needed other colors mixed with it (not all did), what those colors should be, and how they worked with other blacks, grays, and colors around them.
While I'm on the subject, I'll talk about grays. There are a couple of ways to skin that particular cat, and it helps to have a sense of how they'll each look in print. One way is to use only black ink printed in tiny dots of various sizes to give the illusion of different shades of gray, as in a black-and-white newspaper photo. The other is to mix cyan, magenta, and yellow inks (plus black for darker tones) to produce a gray.
In the image below, the top left square and top right square look like the same color of gray. They're not. The close-up views below show that the left gray box is composed of little black dots, while the right gray box is composed of overlapping cyan, magenta and yellow dots.
While the grays look interchangeable on a computer monitor, they give you different effects in print. A black-only gray is colder and crisper, while a cyan-magenta-yellow gray is warmer and softer. It's a hard difference to describe but can be quite apparent on the page, especially when two different types of gray are next to each other. I had some trouble with that in Mom's Cancer, and tried to turn it to my advantage in WHTTWOT as a subtle effect I could deliberately wield where I wanted.
We'll see how it all works in actual print, and if it was worth the effort.
I understand Neil has already transmitted some final files to the printer. We'll still have opportunities to make changes to the proofs if needed, but the book is essentially locked down now. A milestone achieved.