My print experience so far is limited to newspapers. I haven’t seen many colourspace mistakes in other types of printed products. Usually an artist gets in a hurry or is distracted and allows a piece of art to get out without converting it from RGB to CMYK. Preflight functions in software packages are supposed to catch those errors and notify the user in a report but that’s never 100% reliable. So occasionally this kind of error makes it all the way to the press. Once it’s discovered, it’s my job to fix those problems so that the job runs correctly.
Someone asked me in a meeting recently what the difference looked like. I told her that it looks like you’re seeing the image behind a dirty window screen. It’s not as clear or sharp and looks as though the press is running out of ink in that particular spot. Below I have a couple of examples of what this looks like.
This is a “normal” image. When it has been converted to CMYK and printed in 4-color process it should look like this. It should be clear and sharp. The inks should not look watered-down.
This is a photo with the Black channel missing. This is what happens when you attempt to print an RGB file in 4-color process. The Red, Green and Blue channels in the RGB image convert to Cyan, Magenta and Yellow in 4-color process but the black is missing.
When you run across these types of errors they should be fixed immediately. Sometimes deadline pressures don’t allow for that. If that is the case, do the conversion anyway to make sure it prints correctly the next time that art is used.
Colourspace errors are not limited to raster images. Vector graphics and type often end up on the press in RGB as well. Sometimes a designer will use cyan, magenta and/or yellow in artwork and simply not have black. There might be green type that is composed of cyan and yellow or a red that is magenta and yellow. Intentional design elements are not errors. Experience will give you the ability to know the difference.