The $\text{RGB}$ cube model represents colours as vectors in a three-dimensional vector space, with red, green, and blue as orthogonal basis vectors. A triple $\text{rgb}(r, g, b)$ tells us how far along the red, green, and blue axis our colour is. Pure black is $\text{rgb}(0, 0, 0)$ and pure white is $\text{rgb}(1, 1, 1)$.
Two common ways to parameterise colour properties are the $\text{HSV}$ and $\text{HSL}$ models. We can use $\text{hsv}(h, s, v)$ triples to specify hue, saturation, and value of a colour, or $\text{hsl(h, s, l)}$ triples to specify hue, saturation, and lightness of a colour. Value and lightness may at first seem similar but represent different properties of colours in the colour cube.
Achromatic axis and “spectral colours”
The diagonal axis through the cube, between the black points $\text{rgb}(0,0,0)$ and the white point $\text{rgb}(1,1,1)$ represents the greyscale of all fully desaturated colours, the $\text{achromatic axis}$. Along the edges of the cube that touch neither black nor white are the colours with full saturation ($s=1$) and full value ($v=1$).
HSL
Here we see the HSL colours with lightness = 0.5:
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