What the problem actually is
A monitor produces color by emitting light: red, green and blue pixels (RGB) that combine in the eye. Print produces color by reflection: inks or toner (typically cyan, magenta, yellow and black — CMYK) absorb part of the light and reflect the rest back to the eye. These are two different physical phenomena, not two variants of the same color.
RGB and CMYK don't cover the same color space
The color space reproducible in RGB (gamut) is wider than CMYK's in several areas, particularly very saturated blues, greens and oranges. A vivid color that looks perfect on screen may not have an exact equivalent in print: the system substitutes it with the closest available color, and it's this substitution — not an error — that creates the perceived difference.
Always convert to CMYK before delivering the file? It depends
There's no single rule that fits every production workflow. Some printing processes (especially traditional offset) genuinely require a file already converted to CMYK with the correct profile. Others — many modern digital presses — work internally in RGB with an associated ICC profile and handle the conversion at the RIP stage: forcing a manual conversion before delivering the file can, in these cases, produce a double conversion and a worse result, not a better one.
The correct choice depends on the specific printer's workflow, not on a habit applied regardless.
What ICC color profiles are
An ICC profile describes how a device (monitor, printer, press) interprets and reproduces color. Embedding it in the file, or declaring it explicitly, lets the software and the printing equipment know what reference the colors were designed against, reducing — not eliminating — unexpected differences.
Why the monitor is never a reliable print proof
Even a calibrated monitor remains a device that emits light, not one that reflects it: it shows an approximation, however accurate, never the real physical result. A certified print proof, made with the correct target profile and viewed under standard lighting conditions, remains the only truly reliable reference before an important print run.
A practical check before printing
Check the file's working color space (RGB or CMYK) and whether it has an embedded ICC profile. Check for isolated RGB colors left by mistake in an otherwise CMYK file (often text or elements imported from other sources). If you're not sure which workflow is required, ask the printer which color space and output profile they use before converting anything manually.
Cases where the difference is more noticeable
Very saturated blues and greens, fluorescent or metallic colors (which require special inks anyway, not reproducible in standard CMYK), skin tones in portraits (very sensitive to small variations), and large flat color fields, where even a minimal color drift is more visible than on a complex image.
Common mistakes to avoid
Judging the final color by looking at an uncalibrated monitor or under incorrect ambient lighting. Converting the file to CMYK with a generic, arbitrary profile instead of the one requested by the printer. Ignoring a profile already embedded in the file and converting it a second time unnecessarily. Expecting an exact, pixel-for-pixel match between screen and paper: physically impossible — the realistic goal is a perceptually faithful match, not an identical one.