Many of my Muslim readers, now in the throes of Ramadan, their holiest month when eating is prohibited daily from dawn to dusk, are probably really bent out of shape. Their main test of when to cease eating each morning is by distinguishing a white thread from a black one.
As for me, one evening, had a similar battle when hooking up a digital-to-analog converter (using red-yellow-white A/V cable) to a VHS VCR, in turn to a analog TV. The portion with the yellow plug is for video, that with the white plug is for audio. The red plug, also for audio, was here irrelevant due to the monaural nature of this VCR (it would typically be used for stereo purposes). So anyway, due to the A/V inputs (and outputs) being in a deep nook in the rear face of the VCR, without enough illumination, the yellow and white jacks were hardly distinguishable.
For this situation and the Ramadan test above, the same phenomenon is at play here. Colors cannot be distinguished until light is bright enough. Until that threshold is reached, the retinal rod cells all act the same way (though differently with different colors, I assume these are different shades of gray). But with enough light, yellow and white are distinguishable, and ditto for the daily Ramadan thread test.
Hmm…maybe I should use a flashlight next time. As for the early-morning “thread test” and the all-day fast throughout Ramadan, that makes me glad (personally at least) that I’m not a Muslim!