The above are examples of colour mixing. The colour wheel in the top left hand picture uses Staedtler Karat aquarelle pencils. Gouache and acrylic paints were also used to experiment with mixing complementary colours to produce browns and greys and a range of tones with the addition of white or a cool or warm dark.
The bottom picture mixes indigo, sepia. This combination is recommended in Hornung D(2004) Colour, A Workshop for Artists and Designers on page 45 to mix a warm and cool dark which can be used to darken colours and mix muted colours and chromatic greys. Adding white to the mixture helps to identify whether it has a cool or warm bias.
Using the warm and cool dark clearly gave more interesting muted colours than the addition of black which seemed to dull the outcome.
In addition, I absolutely loved the beautiful blue-gray tones that adding white to the sepia & indigo mix gave and couldn’t resist painting with them.