Mixing greys to provide the subtlety required in oil/acrylic painting cannot be achieved by simply mixing black and white together. Try this for an idea. Take the three primary colours; Red, Yellow and Blue – which particular red, yellow etc will affect the outcome so experiment – and mix them together. Use small amounts of paint and try to mix equal amounts. You will end up with a sludgy colour; now add a little white and bingo a grey appears. If you want a cool grey add a little more blue than the other two colours, for a warm grey a little more red and so on.
An endless range of ‘greys’ can be achieved, for example, using Alizarin Crimson as the red, will give a different grey from using Vermillion. If you want a more sophisticated approach try to find out whether the colour you are using is a cool tone or a warm tone, then just mix the cool tones or the warm tones. For example, Permanent Rose is a cool red – slightly bluish, mixing it with ultramarine and a cool yellow like lemon yellow will give less sludgy grey.
A really good exercise in colour mixing would be to produce a whole painting just using greys, cool greys in the background, warm greys in the foreground and a huge range of tones of grey in between might be a good starting point.