My original idea was to take a shot of the blue screen of my television but dismissed this because of the reflection in it. The next idea I had was the post box just down the road which fulfilled the criteria of a strong definite colour:-
Out of interest I measured the RGB scale at the top of the shape in the letter G. In the “correctly exposed” F5.6 shot it was 184. At F4 255; F4.5 252; F6.3 180 and F8 151. The minor difference between F5.6 and F6.3 could be due to a change in daylight values?
In any event the red in the F4 shot is far weaker and not as striking as the F5.6 shot but, as might be expected, the F8 shot provides a much deeper, vibrant and somehow more solid red.
I thought I’d try the exercise with Green and Blue to see if there was a noticeable difference in the change of colour with the same range of exposures. For the green I used a leaf from the garden. It had a fairly reflective surface and its interesting to see with the underexposure at F8 the differences in the reflecting and less reflecting areas is not as obvious:-
Of the shots above I think, to my eyes anyway, that the F6.3 shot is probably the most accurate as the 5.6 one is a tad too light (and more reflective in terms of highlights) although it is a close run thing between this and the F8 shot. This seems to echo the red pictures above
For the blue I used a nylon jacket :-
I this case I think the 5.6 shot is pretty close but again its close with the 6.3 shot.
As to what can be inferred from this test (albeit a very rough one) I think there may be an argument to say that variations of exposure from the norm may be appropriate depending on the colours involved. It is the case that there are a loot of variables involved here, the camera takes an average meter reading , light may change, reflectivity etc. The answer, I guess is to experiment both at the point of taking the shot (assuming time permits such experimentation) or in post production.
It also shows that my recollection of colour is not good as memory (well mine anyway) is an unreliable guide. To combat this shortcoming I often use (although not as often as I should) this little accessory:-
This enables me to make accurate white balance settings (when I remember) and create colour profiles in lightroom for my cameras as well as being a handy point of reference for colour.