If you feel like experimenting a little, get a sheet of paper and some watercolours (red, blue, yellow). I am sure you will understand the science of hair colour wheel better by doing so.
Here is the formula for hair dyeing:
Underlying natural pigments + artificial pigments = result colour
By dyeing your hair you can enhance, soften, neutralize and change the colour of both natural and dyed pigments.
Did you know that if you mix red, blue and yellow you will get brown?
Brown is the base colour that can be changed regardless of the deepness of the colour. (dark brown, brown, light brown, dark blonde etc.)
Red + yellow = orange
Red + blue = violet
Yellow + blue = green
If you know what colour the underlying natural pigment in your hair is than paint it on the paper in a circle. Experiment how colours change. It works the same way if you want to change a shade of colour ( from copper to natural) or you want to darken your hair. For example never dye your coloured yellow blonde hair with ash shade because you will end up with green-yellowish hair. (yellow + blue = green)
I have shared it before but I am going to share it again:
Meaning of colours in hairdressing:
Blue – ash
Red – mahogany
Yellow – gold
Green – matt – natural