HSV and HSL (or HLS) colorspaces are practical models which are not related to any specific material. They use the hue (H) system to code colors. To understand hue let imagine a circle where red is at position 0°, green at position 120° and blue at 240°. Obviously, by a rotation of another 120° step, you come back to red (because 360° points to the same  position as 0°).

Now let imagine that between each of these points you do a progressive blend of the two adjacent colors. You observe a continuous transition between all possible colors (with yellow at 60°, cyan - a sort of light blue - at 180°, and magenta - a sort of purple - at 300°).

Saturation (the S component) is, roughly speaking, the inverse of the amount of white added to the color (that is when S = 100% no white is added to the color; for example pink is an unsaturated red).

But the two models differ on the third component.. V, for value, is a measure of the brightness of the color.

For example a pure red 255 is H = 0, S = 100%, V = 100% (when these parameters are coded on one byte, you may find 255 instead of 100%). When S = 0 and V = 100% you obtain white whatever the starting color (hue) is.

On the contrary L (lightness) behaves in a different way: pure red 255 is H = 0, S = 100% and L = 50%. Between 50% and 100% you add white to the color: for example H = 0, S = 100% and L = 75% correspond to R = 255, G = 127, B = 127. When L is 100% you get white, whatever the color chosen for hue. Therefore L interferes with S for L > 50%. Here again you can find 255 in the place of 100%.

Most of the fractal programs use the RGB model to define colors but, in addition, some of them can also use the HSV model (Fractal Zplot) or the HSL one (Ultra Fractal). These two models are very useful to build gradients in palettes used to color the fractal images (HSV, in my opinion, is the most intuitive one). But in all the cases the program must translate the H, S, V or L parameters to RGB, which is the only system used by graphic cards and computer monitors.

   For more information:  Read The HSV Colorspace