Interactive color wheel. With JavaScript enabled you can drag a marker around the ring, or drag the hue slider beneath it. The slider is the spectrum in its straight line and the ring is that same line curled shut, so the two always hold the same hue, shown in the centre as its angle from 0 to 360 degrees, where 0 is red, 120 is green, and 240 is blue.
A full color wheel can carry a second dimension on top of hue. Going around it is hue, the angle. Going inward, toward the center, is saturation: out at the rim each color sits at full strength, and as you move in it fades toward white, which is why so many wheels have a pale hub. The wheel above keeps to that rim on purpose, every hue at full strength, so nothing pulls your eye off the angle, the part worth getting comfortable with first. Saturation is a story of its own.
The ring itself is usually built from a few anchor hues with their blends filling the space between. Three do most of the work, sitting a third of a turn apart, and the colors in the gaps are simply those three mixed. Land exactly between two anchors and you get their direct blend; step to either side of that and you get finer mixes again. It is the same move repeated the whole way around: a handful of pure hues, and everything else is the measured distance between them. That is the trick of the wheel. It turns mixing, which is fiddly, into position, which is easy to see.
Once hue is a position, every relationship becomes a direction. Colors that sit side by side share a mood, because their wavelengths are close. Colors straight across the ring clash the hardest, the high contrast you reach for when one thing has to stand out. Space three out evenly and you get a balanced set. None of that can be read on the straight spectrum, where across from means nothing. The circle is the thing that makes distance meaningful.
You do not have to hold any of this in your head. Open the Codigrate color tool, drop in any color, and the number it labels hue is exactly that angle, the color's seat on the ring from 0 to 360. The quietly strange part is that the figure was a measurement of light the whole time, just curled into a circle so you could point at it.