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But doesn't a color simply consist of levels of Red, Green, and BlueNo, not at all. Pantone seems arduous because you're unaware of all the simplifying assumptions that you're otherwise making.
Your perception of a color for a single illumination stimulus is indeed a simple three-dimensional quantity (as your eyes have three color channels). But the stimulus itself is defined by an intensity at every wavelength.
For an extreme example, something that fluoresces takes in say UV light and outputs lower wavelengths. I think the Pantone model simplifies away things like fluorescence as well [0], but I've thrown out the example to show how many dimensions we're actually dealing with.
For a purely diffuse example, first take a look at https://en.wikipedia.org/wiki/File:Cones_SMJ2_E.svg . To my approximate reading, 530nm and 575nm have equivalent responses for your "green" color receptors. If we have one material that does not bounce 530nm and one that does not bounce 575nm, they can look the same if illuminated with equal amounts of each [1] but then look different if illuminated with a "same colored" source that is lacking 530nm. This is why people have problems with fluorescent and LED lights - colors look weird because their emissions are "peaky" and not the smooth "black body" curve we're used to.
[0] Although maybe not, with the prevalence of whiter-than-white papers, laundry detergents, etc.
[1] With each material having the appropriate amount of red/blue bounce to equalize those channels as well.