Every object reflects different frequencies of light to different degrees, as I imagine you know. If you think of the range of frequencies in the visible light spectrum, you can construct for any object the “spectral power distribution” (SPD) that says how much light is reflected for a given frequency. So far so good.
But we don’t see “frequencies” of light. An object that reflects only yellow and an object that reflects red and green but no yellow will look the same to us, despite the fact that they are reflecting completely different light, thanks to the way humans perceive light. In fact, there is an infinite variety of different SPDs that to humans look like exactly the same color — called metamers — for any given color we perceive.
A color is an equivalence class, not a point on a spectrum.