From the standpoint of a practicing painter, the difference between saturation (color purity) and chroma (color intensity) is fairly academic.

You’re certainly right that mixing paints almost always results in a color that is lower in chroma. For that reason, a three color palette has a limited gamut of available chroma.

I used to think you could buy your primary colors; red, blue, yellow, white, and black, and you could mix any color you wanted.

But it wasn’t until I came across a set of paints that actually listed the CHROMA of the color on the tube that I realized that the reason I couldn’t get really vivid colors was because you couldn’t mix a yellow with a chroma of 12 with a blue with a chroma of 8 to get a green with a chroma of 14.

Whereas according to saturation you could mix white into it and you’re not decreasing the saturation but you are decreasing the chroma.

Correct me if I’m wrong.

Many colours reflect light of high saturation, but only those that reflect light of high colorfulness (high saturation AND BRIGHTNESS) have high chroma. RGB colours that emit maximum saturation red light range from very low chroma (nearly black) through moderate chroma (dark ruby reds) to very high chroma (bright red or R 255). The importance of this concept of saturation for painters comes from the fact that when a coloured surface turns from shade into light, the colours of the light reflected from it tend to follow a uniform saturation series such as this.

I don’t think saturation in HSB is bad. I do think that, for the purpose of an artist mixing paint, the Munsell model, in which the scale from minimum to maximum chroma varies depending on hue and value, is much more useful. In HSB, I can set chroma to 100%. If I then set the value slider to zero (black), the chroma is still at 100%. But there is no chroma there; the 100% is an artifact of the HSB model, in which 100% reflects maximum chroma at whatever the current value is.

In Munsell, at a value of zero, the only possible chroma is zero. That makes a heck of a lot more sense to me. It also better fits a subtractive color model, which is what we’re dealing with when mixing paint. As hue and value change, the range of possible real-world chroma changes. HSB is perfectly appropriate for computer software and other technical purposes. The considerations involved in designing computer interfaces and calibrating monitors have nothing to do with painting pictures, however. So a different model, more attuned to the messy realities of actual human color vision, is better for that purpose.

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http://www.huevaluechroma.com/012.php