This is an appropriate time to bring up the subject of “trapping.” (The word serves a double purpose because it’s one of the greatest hazards lurking in the desktop publishing shadows.)
This is only an issue with printing on a press and not digitally.
Simply put, trapping is the small overlap of colours or screens that prevents ugly white gaps between them. A prime example would be a border around a graphic or photo that you want to actually touch all the way around.
Trapping means that the red graphic extends part way into the black border, but NOT all the way or there is a danger that it will “leak” out the other side! Usually the dark colour “traps” the lighter colour. Trapping also occurs when outline type is filled with a colour or when screens butt up against lines, graphics, etc.
The thinner the border or outline, the more carefully the trap must be addressed. Even more care is needed when trapping two light colours because where they overlap a third colour is created.
Most decent quality graphics programs allow you to do trapping before outputting your separated artwork.