All of the above answers are great, and very informative. But they are also technically wrong, in certain conditions. Once you understand them, you'll be able to understand this explanation of why.
The canonical answer is that cones are used for color perception in bright light and rods are used in low light. But rods have a peak color sensitivity that is very distinct from the cones (see the chart posted above). And more importantly, there are light levels at which both rods and cones are equally functional for color perception.
This is known as the "Purkinje effect" or "Purkinje shift". Basically, when light levels dim, your red color perception diminishes first, but your blue color perception is enhanced (or at least doesn't diminish nearly as fast). The specific effect is that red objects get darker much faster than blue ones. But the brain isn't yet just perceiving the blue objects as a brighter gray, so it seems there is some color perception built into the brain based on the rods.
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