The short answer to “RGB or CMYK?” depends on your printer. But the real question—why your printed cover looks darker than your screen—has a universal cause.
The Printer-Specific Rule
Amazon KDP paperback: CMYK is what Amazon asks for; RGB is accepted. KDP’s own cover guidelines say images “should be in CMYK color to ensure your cover looks good in print.” An sRGB PDF still goes through — Amazon converts it at print time — and plenty of authors upload RGB without trouble, but you are then trusting Amazon’s conversion rather than seeing the shift yourself. A proof shows the shift either way.
Kindle ebook: RGB. The ebook cover JPEG must be RGB; a CMYK JPEG looks dull on every screen.
IngramSpark: CMYK is Mandatory. All images in your IngramSpark cover file must be in CMYK. Their barcode also requires a specific pure black ink mix (0/0/0/100). The best practice is to convert your RGB master file to CMYK only as the very last step before exporting your final PDF.
Lulu: follows standard PDF workflows. When in doubt, use the precise template they generate for your book’s specifications.
The usual practice: the master file stays in RGB, the conversion to a printer’s color space happens once at final export, and nothing is converted back.
The Real Reason Prints Look Darker
Your screen creates color by emitting light; paper creates it by absorbing light with ink. The RGB spectrum includes vibrant, saturated colors—think electric blue or neon purple—that simply cannot be replicated by any combination of printing inks. When your file is converted for print, these unreachable colors are mapped to the closest printable alternative, which is invariably duller and darker. This isn’t a flaw in any one printer’s process; it’s a limitation of physics that affects every print job.
What this means for your book cover:
- High-Risk Colors: Deep saturated blues and purples, hot magentas, and bright gradients will lose the most vibrancy. Expect them to shift noticeably.
- The Darkness Multiplier: A moody, midnight-blue scene on screen can print as nearly solid black. If your design lives in the shadows (roughly the 10–25% brightness range), consider lightening it before you finalize the file.
- A proof copy: no on-screen soft-proof fully shows how the artwork will look on paper.
A Note on “Muddy” Solid Colors
Large, flat areas of a single rich color (like a solid burgundy back cover) can sometimes print with a slightly uneven, blotchy appearance on print-on-demand digital presses. This is because POD uses toner or inkjet technology, not the offset lithography of large print runs. Using a textured background or a gentle color gradient can help hide this effect better than a perfectly uniform fill.
Quick Reference Chart
| Printer | Required Color Space | Who Handles the Conversion |
|---|---|---|
| Amazon KDP paperback | CMYK recommended; sRGB accepted | Amazon converts an RGB file at print time |
| IngramSpark | CMYK | You, during PDF export |
| Kindle Ebook | sRGB (never use CMYK) | n/a — all screens use RGB |
For the other half of cover preparation—getting the physical dimensions, bleed, and spine width correct—refer to the cover size requirements guide or use the automated calculator.
Sources
- KDP Community: print cover in RGB — KDP accepts RGB, converts internally
- IngramSpark: File Requirements for Print Books — CMYK requirement
- VeritasCanvas: RGB or CMYK for KDP covers — conversion behavior and gamut discussion
