There are a few different ways you can mix neutral colors. You can get interesting greys and browns with just the slightest hint of their base colors by adding complementary colors together…
Neutral Tint Watercolor - DANIEL SMITH Artists' Materials
How to mix interesting neutrals and greys with watercolor
Watercolor: Mixing gray
Colour Theory – Marion Boddy-Evans
Sarah Kaiser-Amaral Art
Painting with only 2 colors! How to work with grays — Café Watercolor
Mixing Your Own Gray in Watercolor
What Colors Make Gray? Color Mixing with the Primary Colors
Lessons in Layering with Watercolour: an online course with Natalie Martin