I'm a pre-press worker as well as a graphic designer. I often encounter this problem when I use the Eyedropper Tool to sample a color in order to create a bleed.
On the monitor, colors show exactly same. However, the colors turns very different after being printed. (e.g. original dark blue, sampled color: light blue)
Does anyone know whats actually happening?
It all depends on the actual application.... for example Shift-clicking with the eyedropper in Illustrator or InDesign always samples the screen preview (RGB) not the actual object. (InDesign actually throws up a warning telling you this) So, the monitor calibration plays a huge role in how accurate any eyedropper click may be.
It's better to check numbers if you can't directly select a colored object. Hover your cursor over an object and look at the Info Panel (in most apps). You'll see a readout of the color breakdown, use those numbers to create a new swatch/color.
Or check your numbers after using the eyedropper and applying the new color to ensure they match.
check your separation preview to see the exact print registration of colors. check one by one the overprinted shapes or text. maybe they are overlapped or overprinted.
