It’s important to know how to mix greys because they provide context into which more saturated colours can sit. Normally, you want the saturated colours at the focus of attention in your painting, not everywhere. In nature backgrounds typically “fade to grey” because of atmospheric perspective.
So here are some ways to mix neutrals and greys in watercolour. My favourite is Cerulean + Magenta + Yellow. This is like the CMYK model of inkjet printers. Basically, you can mix any colour triad equally spaced on the wheel. Or, you can mix two complementary colours, but they have to be exact opposites. In this case it might be harder to make it perfectly neutral.
I painted these swatches in Windsor & Newton watercolours. It took me a while to figure out that the W&N “Windsor” Blue and Green are the same as Phthalo Blue & Green.
As an exercise for mixing grays and neutral tones I did a study of two bottles, pictured below. The background is actually Lamp Black so I cheated a bit. All the other shadows are built up from colour mixes.