What it means
RGB (red, green, blue) is how screens make colour: three lights added together, white when all are on. CMYK (cyan, magenta, yellow and key — black) is how presses make colour: four inks that each absorb light from white paper, darker the more you add. Every photograph, every cover mock-up on a monitor, is RGB; every printed page is CMYK.
The two do not cover the same range. RGB reaches saturated blues, greens and oranges that no mix of four inks can reproduce, so a vivid RGB cover converts to a duller CMYK one — the classic "it looked brighter on screen" complaint. Converting the file yourself (with a proof) shows the shift before the printer's automatic conversion does it silently.
Two CMYK habits matter for books: set body text as 100% K only, never four-colour "rich" black (which mis-registers into fuzzy edges), and keep total ink coverage under the printer's limit, typically 240–300%.
Make the book, not the checklist
Describe a book or upload a manuscript. Bindery writes with you, typesets in LaTeX, sizes the spine from the real page count and produces printer-validated files — the watermarked proof is always free.
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