This is an exercise you should try if you're interested in colour mixing. I did this during my watercolour class a couple of weeks ago and it has really helped me understanding tone and colour better. What you do is mix blues and browns. Look how many earthy colours I created by using nine different hues. You can for example get a light greenish colour by mixing Prussian blue and Naple yellow, or an almost black colour by mixing Sepia and French Ultramarine.
On the top two rows you can see the colours I used and below you can see all the swatches (and you can do more than that). All are made with Winsor and Newton professional watercolours.
Blues: Cobalt blue, Prussian blue, French ultramarine, Indigo
Browns: Naples yellow, Raw umber, Burnt umber, Brunt sienna, Sepia
Image from Tate.
Image from pablopicasso.org.
What's also interesting is how you start noticing this palette in other artists' work. For example Braque and Picasso's blue period.