@icon_of_computational_sin@mstdn.starnix.network
Are you playing this card with me, seriously? Okay, let's go…
You are comparing two very different things. In case with fiction books you are not supposed to navigate within a page easily, maybe vaguely remember where it was on the page, unlike with code.
When you are editing printed text on the other hand, or compiling quotes, so you can find and use them later — in other words things that require easily finding the precise location on the page, very adult people, who mean serious business :marseysuit: , start using stationary that comes in bright "childish" colours: those acidic markers — often several at once on the same page, colourful arrow-shaped stickers, etc.
I mean, you can probably get away without any of this, but why would you? To appear grown up and cool like those anime schoolgirls drinking canned black coffee from vending machine?

Even before colour printing was invented non-fiction books that contained well-structured text started relying on fonts with different weights and cross-hatched shapes — to make up for the absence of colour. Which wasn't easy with technology of the time, no one was doing it for fun. Nowadays colour printing became more affordable — not to the point of ubiquity, but you can already find expensive editions of technical books using colour.
You can have personal preference and that is okay, but never dismiss colour as something unimportant and purely decorative — it's as good a visual anchor as any other means.
By the way, does your "adult" act sit right with using nyan-cat as progress indicator? No, I'm not leaving you alone with that one! Is it animated? Would be cool if it was — you know, for that "strictly work" focused mode when you want no distractions :marseysmug3:
Your card is beaten, sir!