To make a brown add any two opposites in the colour wheel. Ie. red and green, blue and orange, or yellow and purple. To make a grey add blue to brown and add some white. This is particularly useful In practice when you want to take the edge off a colour or grey it out a little. First, add very small amounts of the opposite colour to desaturate it, and add blue and white to take it towards grey. Unless you want to make it grey, go very carefully with the amounts you add. I sometimes jokingly say to use homoeopathic quantities!