I. “Turner’s favourite colour was yellow. He spent hours studying its myriad iterations, using more yellow pigments than any other …

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Turner admired yellow’s optical power. Bright and warm, it jumps out at us from a distance and forces itself on the retina … Turner believed that yellow was the foremost of the three primary colours because it was the closest to white, to light, and therefore to the sun. … Though he wasn’t a straightforwardly religious man, Turner was a lifelong worshipper of nature. The whole world, he thought, was bathed in a divinity that originated in the sun.

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In his unfinished “Norham Castle, Sunrise” (c. 1845) Turner didn’t paint a solar orb directly but invoked it with a cloud of lemon-posset yellow. For all its lyrical beauty, the painting has a curiously destabilizing effect on the eye, oscillating uncomfortably between visibility and invisibility. … That is because Turner’s sun and sky are insoluminant – they are equally bright, which creates mayhem in the visual system. To the part of the brain that mostly processes luminance the sun is invisible, but to the part of the brain that mostly processes colour it is easily distinguished from the blue sky around it. … his sun overwhelms our visual apparatus just like the real one.

The most potent of all Turner’s suns is “Regulus” … More arson than artwork, “Regulus” is almost too incandescent to look at. When it originally went on display in London, critics advised the public to shield their eyes to avoid injury … In one sense Turner hadn’t simply painted the sun but recreated it… He, like Prometheus, had stolen fire from the gods and so become divine in his own right.”

James Fox, “The World According to Colour: A Cultural History”

II. “The elements of visual art have long been held to be color, shape, texture, and line. But an even more basic distinction lies between color and luminance. Color can convey emotion and symbolism, but luminance alone defines shape, texture, and line. ‘Colors are only symbols,’ Pablo Picasso once wrote. ‘Reality is to be found in lightness alone.’

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A monochromatic rendering of “Impression, Sunrise” reveals that Monet painted the sun at exactly the same luminance as the gray of the clouds. If he had rendered it in a strictly representational style, the sun would have been brighter than the sky by a factor too large to have been duplicated with pigments. If he had made the sun lighter—which is closer to the way it would appear in reality—it would have lost its quavering luminosity and would have seemed, paradoxically, less bright. Rather than appearing as a source of light, the sun would have looked like a cutout affixed to the clouds. By rendering the sun the exact luminance as the sky, Monet achieved an eerie effect: his orange sun appears to pulsate across the
grayish-green water.”

Margaret Livingstone, “Light Vision” via https://switkes.chemistry.ucsc.edu/teaching/CROWN85/literature/lightvision.pdf