There are two exercises you can try to help train your eyes to really see every colour possible in something you are painting. They are both still life projects.
The first is to create a still life made up of as many white objects you can find and obviously place them against a white cloth or background, so everything you see is white! But is it? There are shades and tones of white in front of you, try looking at the white objects in a different way: is that a cool white with maybe some blue in it, against a warm white with maybe a touch of orange or pink in it? Of course these shades of colour are very subtle indeed and so you need to mix the colours in a very subtle way. Before you know it you will be observing a whole range of colours and tones amongst all those whites. The second exercise is to do the same again, but with all black objects AND you are not allowed to use any black paint! You will need to use a whole range of colours to mix your various shades of black, some warm and some cold, etc. Once you have managed to complete these two still life pictures, you’ll start to look at colour differently, for instance flesh tones and all the various warm and cool colours you can see when doing a life drawing. Have fun!