This is Drew Struzman's back cover to Sabbath Bloody Sabbath in 1973. I'm specifically asking about the use of blues and some alternative texturing in them (like in the head of the man on the left side). I know he began with photo references, and I'm assuming he did some sort of alternative processing treatment to them, rather than just drawing it all up in colored pencil from the references and spewing the color scheme completely out of his brain. Because virtually all of my design knowledge comes from the digital era, I don't know how else one might achieve color treatments like this.
In 1973, it was hand drawn/painted.
There's no reason to believe he didn't merely draw it all. And there's especially no reason to believe he didn't specifically hand color the image how he wanted. It's kind of insulting to him to think he couldn't have possibly drawn and colored the image. Of course he could have. That's what artists did in the 70s before everyone with Photoshop wanted a filter to complete work for them.
Prior to the 1980s almost everything was hand drawn. At most a photo may have been loosely traced for an outline to get size/position correct, but beyond that there was zero "photo processing" for creative illustration. It was all the artists hand and eye.
Typical processing was to draw and sketch on translucent layout vellum, then overlay new sheets, and trace your work again refining, repeat until you are happy. Then transfer the line art drawing to a more substantial paper, such as bristol board, with something like an artograph and start working up the final image in whatever medium was to be used. Color studies may have also been performed on the vellum preliminaries.
I don't fault you for thinking there was some sort of "photo processing" - it's almost the standard nowadays. So much more is typically explored if, after getting size/position, you stop looking at any photo. Or you just don't use photos for anything other than an initial reference for hand creation. The computer, it's software, and photo processing overall are terribly limiting in terms of artistic creativity in my opinion. They always have been. As a final polishing tool the computer can be great, but as an initial idea tool, it sucks.
Ah the 70's. The beautiful age of colored pencils and aerographs.
In the 70's (I think at the end of them) there came two books you should check out. One is by Boris Valejo Dreamland and second by Chris Achilleos Sirens. Both with artist commentary and some snippets of processes they do.
For example first they sketch the idea, Struzman's was hard into symbolism back then (Ernie Cefalu had the idea of using two color pictures in case of Sabbath), then they would take photos of models with different light and then combine them in one picture (cut and copy, multiple exposures and so on).
On how to achieve color treatment - first you need to have an idea on what is the "main color", for example in Sabbath Bloody Sabbath it's yellow/red on the front cover. Now you go to color wheels, colors books and you look for other ones. What you have in total opposition? Blue. What is a color in beetwen? Green.
Look here Blue and Yellow color palette
Back then it was easier to buy a color wheels books or to create ones by yourself. (you take a van Gogh 535 and paint it diluted with turpentine from 5% to 95%, and then you mix it with yellow and so on). It was analog "cooler" (or what's the name of that thing in photoshop)
If you look at digital artists processes videos you will notice they usually start with establishing base colors then they use color wheels to find complementary ones (complementary is, I think, the right word. Not in opposition but ones that work well with one). Almost all color theory guides I've found on pinterest showed similar process.
Summarizing - you learn how to get such color treatment from your brain because you studied and looked at colors for hours and hours.