Your eye has three types of receptor cells, the sensitivity of each type peaking in different spectral regions. Roughly speaking, there's one that peaks in red, one blue, one green. (It's not quite so clear cut, but your brain is really good at sorting out messes like this!) When you look at a fire engine (assuming it's red) It's mostly the red receptors that are firing. If your fire engine is white (as they are in New Haven, CT), all three receptors are firing. For pink fire engines (Yes, they exist! I saw two last month! If you don't believe me, ask Google.) the red receptor is firing full-on, but the green and blue are on too, but not full-on. Maybe 80% on. Almost white, but biased toward red. Your brain takes that in and calls it pink.
BTW black is no stimulation of any of the receptors. Brown corresponds to very little stimulation of any of the three types, but of what there is, the reds are firing more than the greens or blues. So brown is close to black, but biased toward red.
But it's far from that simple: your brain takes into account the adjacent color before deciding what to labeling the color of the object you are looking at. It is not simply a matter of mixing frequencies!!