Have you ever bought something—say, a shirt—that seemed to be one color in the store but looked totally different once you got it outside? If so, then you, my friend, have been a victim of lousy color rendering: artificial lighting that doesn't accurately represent the "true" color of objects.
In the lighting industry, color rendering is measured as an index from 1—really, almost impossibly, bad—to a perfect 100. Perfection, in this case, is when a light source is functionally equivalent to daylight.
Like sunlight, old-school incandescent lightbulbs are full-spectrum, which means they put out—and can therefore render—every color in the rainbow. Other lights, such as el cheapo fluorescent lights and mercury vapor streetlights, may glow with only three or four colors. With color rendering index scores in the dreary 70s or less, these sources don't show things with their true colors.