When you create design ideas, your eye is the most important judge. The more designs you create, the more trained your eyes will become. But at no point will anyone’s eyes ever become so well-trained, that we could rely on them when we finalize our designs.
Different monitor settings = different colours
The easiest example to understand how our eyes can fool us is using different monitors. The settings of each monitor make designs look different – sometimes only so much, that the eye doesn’t find the difference. But no matter how small the difference is, the hexcode of your rgb colours, and the CMYK values of your prints, do show them.
You can use the colour black or you can use the hexcode #000000 (which is pure black in RGB); the latter is the only safe choice, as it is based on objective math principles, while the former only relies on our subjective eyes.
Use only CMYK for printing
The most obvious example for screwing things up with wrong colour values for designers is: print results that where designed in RGB colours. A typical beginner mistake is to use RGB in printing, as print systems need CMYK values. Even if you get all the hexcodes right, which guarantees the same results with every monitor, you’ll still screw things up, as the printing process can’t use these hexcodes to create the results you want to have.
Adobe programs offer us the choice between RGB or CMYK colour systems. We shouldn't forget to actually use what is needed for a specific project.
Colour consistency when using different programs
When you use Adobe programs, you can synchronize almost everything using Bridge. But when you use various programs of different developers, you should never rely on your eyes only.
The black you’ve used in one program might look like it is still black, but only checking the hexcode (or CMYK values) will guarantee, that colour consistency between the programs are given.
Best practice for designers
Whenever I finalize designs, I add a colour reference guide for clients showing not only what the eye is seeing, but also the exact colour codes/values. This routine doesn’t just help the client (who might not have much of a clue otherwise which colours to use) but it also helps you, when you go back to your projects for revisions or reference.