How do humans perceive color? Is it just the way we see it or is color actually there?
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In our eyes, we have four types of light sensitive cells. There are rods, which can detect a wide range of wavelengths of light and can detect it even when it’s dim. And there are three types of cones, which detect narrower ranges of wavelengths. The three types of cone cell are called Short, Medium and Long based on the size of the wavelength they detect. They’re also called Blue, Green and Red, based on what we perceive when they’re activated. When your L cones are activated most strongly, you perceive red. When your M cones are activated most strongly, you perceive green. When both your L and M cones are activated then you perceive yellow.
In regard to your second question, the wavelength of light is an objective physical property that exists independently of humans. The way we perceive that spectrum as a rainbow is a result of the way our cone cells work.
Edit: Though, actually, the way we perceive color is actually much more complicated than just the raw inputs our eyes provide. It also depends on context. An object that reflects all wavelengths equally will look white in normal light, but if you shine a red light on it, it will reflect that red light and appear red. But if everything looks red, then your brain concludes that the light must be red (like at sunset) and automatically adjusts. That’s how the [checker shadow illusion](https://en.wikipedia.org/wiki/Checker_shadow_illusion) works. The light square in shadow is the same shade as the dark square out of the shadow, but because you can see the shadow, your brain adjusts and the square in shadow looks lighter.
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