As part of my continuing development I use still life as an eye training tool. Don't get me wrong, I also paint still life in the studio for fun and general development, but it is currently not my primary thrust.
So, how can one train the eye to become more sensitive to colour? Part of the challenge is to see what is actually there as opposed to what you think is there. When you become more sensitive to experiencing what is really there you realize that you usually see local colours or you belief of what colour an object is. For example, sky is blue, the blue table cloth in the light is light blue and so forth. The first step to increasing colour sensitivity is to be completely relaxed. Free of tension and anxiety. To help get relaxed I find composing the scene at hand with a thumbnail sketch is beneficial. Once the composition is developed, drawing, values, and composition are out of mind.
The paradox is that you must then become active by painting. A sprinter has the same problem. Perform at maximum power and speed while simultaneously being completely relaxed.
In this relaxed state, scan the scene you wish to paint. Do not linger on objects. Scan from one shape to the other. Compare the shapes over and over. This is the opposite of staring. Scan for a few minutes before beginning to paint. Stay relaxed. You should feel no tension in your face.
Set Up - Aerator and Bottle
Here is a still life set up. It is simple. Four shapes. That makes the relaxation and scanning easier to perform. The shapes can be much simpler, and this along with the light (here I used a day light bulb) can be controlled in the still life studio.
Set Up and Start
Here is a start on the still life set up. The four shapes are indicated in GREEN. While scanning, paint the closest flat colour to each shape. As you continue to paint you can continue to draw. You will also note that the label on the bottle has had a bit of colour added to it as it moves into shadow. These colour notes become visible to you as you continue to scan and compare colours. The more you have filled in the more colours become apparent. Notice the lack of a value seeking underpainting is not used. The idea is that the right colour note will give you the right value. Also note the grey composition mask laying near the setup. The GREEN arrow points to a small hole in the mask. As you move into scanning and correcting the colours first laid down, this tool can be very useful. It is a colour isolator. Viewing a colour through it keeps surrounding colours from influencing the colour in question. Here you can see, even in the photograph, that the aerator has a myriad of colours on it. At this stage you are relaxed, you are scanning, and as you see a colour, you mix it and place it as required. Then you continue to scan. Someone observing you should see you scanning most of the time - look, look, paint, look etc. Remember, the purpose here is to become receptive to colour, not to render.
Mid Way
The exercise is incomplete here. Look at the series of colours on the label, on the bottle, the aerator, and the backdrop. Next steps would include adding an impression of the printing to the label and to continue to add colours as the become apparent. I would stop when I can no longer see colour variations.
This exercise makes you come to realize that for every plane change there is a colour change. In the studio you can control the objects to keep it simple. In the landscape these changes are infinite.