Why blue changes in print: RGB, CMYK, and colour profiles
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If a graphic looks different on a screen and on paper, examine the output conditions before declaring one version wrong. A monitor emits light; paper reflects surrounding light. Devices, paper, ink, and illumination vary, so matching numerical values do not always produce a matching appearance.
Distinguish the colour model from the colour space
RGB expresses colour through red, green, and blue light. CMYK is associated with printing using cyan, magenta, yellow, and black inks. The word RGB alone does not completely define a colour's meaning: sRGB and Adobe RGB are different spaces within that model. See Adobe's colour model guide for the basic concepts.
A colour profile helps interpret what the values in a file mean. Without that context, the same numbers may be interpreted differently. CMYK is also not one universal colour space; the output conditions and profile matter. Simply changing the mode does not solve every output problem.
Make a blog image and a printed leaflet together
Suppose an information graphic with a deep blue heading will appear online and on a leaflet. Preparing the web version in sRGB is usually a practical starting point, while considering support. For print, first ask the printer about the required format, profile, and who will perform the conversion. Avoid converting prematurely to an arbitrary CMYK space.
Duplicate the archival source to make separate web and print versions. A printer may accept RGB and convert at output, or request a specific CMYK file. What matters is delivering a file that fits the agreed output conditions, rather than deciding which workflow sounds more sophisticated.
Do not confuse assigning and converting profiles
Assigning a profile and converting to another profile are different operations. Assignment changes the interpretation of existing numbers; conversion changes numbers to preserve appearance in the target space as far as possible. Check Adobe's colour settings explanation and test on a copy of your source.
An output device cannot reproduce out-of-gamut colours exactly. Soft proofing helps preview specified output conditions on screen, but it cannot perfectly guarantee the actual paper and illumination. For important print work, a small test print is a practical way to check sensitive areas such as a heading colour or skin tones.
Four checks before delivery
Record the file's purpose, embedded profile, output requirements, and final checking method. If small text sits on a dark background, see whether it remains readable after modest colour changes, and do not communicate meaning through colour alone. Numerical codes and a screen preview are not sufficient proof of success.
Perfectly identical appearance on every device is not a realistic aim. Knowing how a file is interpreted and where conversion happens can reduce avoidable changes. Checking screen and print results in their respective environments remains useful longer than merely memorising one colour mode.
Further reading
Original illustrations created to help explain this article.