‘There is a bit of an issue with semantics afoot: when many people say they want to mix a “darker” color, they in fact mean more intense, brighter, higher chroma, etc. Black is dark, but a lighter shade of black is grey. Mixing in black to darken a color will make it more grey. If your object is to tint your color or knock down the chroma, then black is a perfect choice. However, if you wish to maintain chroma while darkening a color, choose a pigment with a similar hue but a wide value range to do the trick’Jess Greenleaf, “Understanding Value Range for Watercolours“
The rule of not using black in a painting, blamed on the Impressionists, ought to have an accompanying rule of not automatically reaching for white to lighten a colour. It’s somehow easier to get to grips with black dulling a colour than realising white does the same. I think it’s because when we’re thinking of a colour we’re got three things simultaneously: the hue (the colour of a colour), chroma (its intensity or saturation) and value (or tone, how light or dark).
It’s simple to lighten the tone of a colour by adding white, and to darken it by adding black, and this is where we generally start. Forgotten is that we’re simultaneously reducing the chroma, or intensity of the colour. It’s perhaps easiest to demonstrate with red. Add titanium white, and you head into pinks, getting lighter and lighter as you add more white. Add yellow, and you head into orange-reds rather than paler reds. And this “not white first” is where colour mixing becomes more complex and interesting discoveries lie.