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> most luminous color

What does luminous mean exactly in this context?

I know that our eyes are most sensitive to green. For example a green LED needs less power to seem as bright as a red LED.

But our eyes don't matter in this subject. So I am scratching my head about what you mean that green is the most luminous color?

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I think this is the color (wavelength) that corresponds to the sun's emissivity peak (irradiance in W/m²).

The fact that our eyes are most sensitive to green is probably not an accident, but is it due to the color of leaves, or is it due to sunlight?

Here is the math used by W3C for computing acceptable contrast: https://www.w3.org/TR/WCAG20/#relativeluminancedef

I suspect luminosity to mean percentage of brightness for a given frequency from a total light source assuming white light. For a more precise definition I would look to physics.

That’s for sRGB, which as for most colour spaces is defined relative to CIE XYZ, which is itself defined in terms of the perception of a “standard human observer”. In XYZ, the Y coordinate is the luminance (brightness) of the colour and that first equation is computing Y from the RGB value

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