Spot colours with a "butt fit".
If the coloured parts of your design overlap each other the overlap will
be obvious and look ugly, but if your colours "butt fit" or meet with no
overlap, paper stretch between prints means you'll often get a tiny
white line between the colours. For some strange reason the human eye
can spot that gap from metres away even if they haven't got their
The standard used commercially in this situation is to overlap your
colours by 20 microns (0.02 mm), that's small enough that the human eye
doesn't notice it but big enough that the printer has a little room to
play and make it look good.
Most printers really will try, by the way. It's a matter of professional
pride, none of them want to admit to producing something that looks bad.
"Those that can do, do do. Those that can't teach at TAFE."
Another design tip: roughly 25% of caucasian males unknowingly suffer a
genetic colour blindness and if you give them an image with red text on
a dark background they can't read it.