CMYK and RGB colour models

Example

A brand with a saturated green, #00B36B, orders business cards and flyers. On a screen the colour glows; on paper the same file comes out a step duller, because ink reflects light instead of emitting it. The answer is not to push the saturation in the artwork. The colour is chosen once for print, and the brand rules carry both sets of values side by side. If that green has to be identical on a box, a bag and a card, it moves to a mixed ink with a number.

Why it matters for a business

The two models describe colour in opposite directions. RGB adds light: a screen starts from black and adds red, green and blue. CMYK takes light away: paper starts white and the inks subtract from it. Some screen colours therefore have no equivalent on paper at all, and a conversion moves them to the nearest thing that exists.

• A print file is built in CMYK with the profile the printer actually uses, rather than left to an automatic conversion at the last minute.
• The four inks together have a ceiling — around 300% on coated stock and less on uncoated. Above it the ink does not dry and sets off.
• A large black area is mixed with a little of the other inks. Black on its own prints grey.
• Web files stay in sRGB. One file cannot serve both ends; two are made.
• The screen is calibrated. Without that, correcting by eye moves the colour the wrong way.

What to ask

• Which profile does the printer work to, and what is the ink limit for this material?
• Do the brand colours exist in both sets of values, and do the two agree when you put them side by side?
• Has a proof been pulled on the real stock before the run?
• Which file is the master that both the web and the print versions come from?

Preparing files for a named printer is part of our print materials work.