Adobe Photoshop is probably the most popular design toolkit amongst “amateurs”, i.e. people reading this blog!  One of the major issues with designing on your home computer or company laptop is that the colors you see on the computer screen or not necessarily the same colors which will be printed.

Let’s look at black as an example.

There are several ways you can print black –  plain black is simply black ink, but you can also print “rich” black which will use other colors as well.  Typically, a combination of the four main CMYK colors are used to create different blacks, and here is where Photoshop will lead a newbie astray; when a plain black color in a file is converted into Photoshop, the resulting black will contain a combination of colors and not just black ink.

Why does this matter?

The problem is that different blacks will be readily apparent when they are printed out – you will easily see a difference between plain and a rich black, which means your design will look shoddy and amateurish too!

The problem is further compounded by how your computer renders any type of black – on the screen, black is simply rendered by an absence of light.  Cool black, blue black, hot black, plain black are all rendered in exactly the same way, and to the designers naked eye, they cannot appreciate any difference between them.  The issue arises when the files are converted into instructions for the printer to use and what appears to be completely and uniformly black on screen is then demonstrated to be different variations in real life.

There are several quirks with Photoshop and how it handles black.  The Edit>Fill command will use not simply black but a full CMYK ink combination.  To use a specified black variant you need to use the Fill with Foreground Color and individually specify the CMYK inks you want to use and the ratios.

The "Move" command also provides scope for a glitch. When Photoshop moves an object, the remaining void is filled with the current background color being used.  If you have a plain black background but the surrounding area is rich black, then on screen it will not be noticed but there will be a difference when it is printed which corresponds to your cutout. 

To catch these and other issues, make it a habit to regularly flip through the CMYK values (use the command-1 thru 4 shortcuts).  This will make sure you don't end up with a black mismatch which you can't pick up from the computer screen alone.