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…
Watercolour Triads - Jane Blundell - Artist
Chromatic Neutrals with QoR Watercolor
Sarah Kaiser-Amaral Art
How to mix interesting neutrals and greys with watercolor
Paint Watercolor Create: Mixing Shades of Gray with Watercolor
Sketching a swallowtail butterfly - video
Painting clouds and the sky - my current approach
What Colors Make Gray? Color Mixing with the Primary Colors
Neutral Tint Watercolor (All The Answers!)