Grays, browns and blacks can be created by mixing complimentary colors which are opposite colors on the color wheel. You can create more interesting neutrals by mixing them yourself.
Here I've mixed ultramarine blue with burnt sienna. Burnt sienna leans a bit towards the orange side which is opposite blue on the color wheel. This combination mixes to create a fantastic range of pale gray to a deep black.
Purple mixed with it's opposite on the color wheel yellow blends to create a neutral tone perfect for painting sand. In this sample I used permanent violet bluish mixed with indian yellow.
Various greens mixed with various shades of red can create brilliant browns and blacks. The third column has a sap green mixed with primary red magenta to create a great shade of brown perfect for a tree. In the last column I combined crimson lake with cupric green creating a deep dark grey.
The colors shown in the top and bottom rows are the pure unmixed colors. The center row combines the color above and below to create the neutrals shown here. I mixed the center squares directly on the paper. These colors can be mixed on your palette. Need a darker shade use more pigment, need the tone to be lighter add more water.
Sure it's easy to purchase specific colors of watercolor paint, but it's more creative and fun to mix your own colors. Do a test swatch of mixing the colors from your own palette that are opposite each other on the color wheel. You will be able to create more colorful and pleasant neutrals, browns, blacks and grays.