Take a look at this image.
Which grey column would you say is darker? The one on the right or left?
Ah, trick question. They're actually the same color. But I'm sure you all saw that coming...
Even when I see it laid out plainly like this, I still have trouble convincing myself of the truth. Our eyes are so tricky!
Here's another example of the same illusion:
These illusions illustrate just how inaccurately we perceive color. Whether we view something as light or dark really depends on the context; background and surrounding colors greatly influence our perception.
Our eyes struggle with all sorts of colors, not just shades of grey. Take a look at the images below. If you didn't know better, wouldn't you say that the small blocks of light purple were the same color in each scenario? Same with the oranges?
Our imperfect color perception is yet another reason why careful proofing is so important. We might like how a color looks in one context, and hate how it looks in another. In the display of pens above, the grey works nicely on the black, white and blue pens. There is good contrast, and the imprint really pops. On the red and lime green pens, however, the grey imprint almost shakes before your eyes. It's a bit distracting and unpleasant to look at.
Don't ever settle for a black and white proof, and pay careful attention to how your selected imprint color looks against the actual item color.