I magine that you are at the Grand Canyon on summer vacation. The sun is setting and you are looking for a place to camp for the night. You turn off the main road and see a sign ahead.
Camping is permitted here.
You shine the car's headlights toward the small sign. However, after the sun has set, your eyes begin to adapt to the darkness. As this happens the subjective brightness of red decreases in comparison to other colors. The fact that this small sign is difficult to read in low light conditions is ironic since those are likely conditions under which people are looking for a place to camp. Sensation and Perception.