Exercise: Control the strength of a colour

For this exercise, I've chosen a yellow framed pictured in a yellow wall as subject. I've used a manual exposure metering instead of the camera's metering to set the average exposure values. I've set an ISO 800 and 1/60 as shutter speed and the metering has given an aperture of f/8. So using such values I've varied the aperture, shooting two underexposed and two overexposed pictures according to my exposure metering. So here are the results:

    

ISO 800
1/60   f/5,6
20 mm

ISO 800
1/60   f/6,7
20 mm

ISO 800
1/60   f/8
20 mm

ISO 800
1/60   f/9,5
20 mm

                         

ISO 800
1/60   f/11
20 mm

So the conclusions I've made after the exercise are that saturation  increases (the yellow is stronger) when I've underexposed the image but brightness decreases. On the other side, when I've overexposed the image looks brighter but also less saturated, so the yellow is washed out.