Voici quelques informations concernant le style fidèle vs style neutre trouvées sur un site CANON:
(Désolé en anglais).

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Faithful Picture Style

When looking at the Picture Styles available in EOS cameras, the parameters of both
Neutral and Faithful look identical, however there is a difference.

While both will produce flat, muted images, if you are shooting products that have a
defined colour (for example the red of an Italian sports car) then by shooting with
the Faithful Picture Style and using lighting that has a colour temperature of 5200K,
the colours in your images will be colourimetrically accurate.

If your client needs you to match the colours of their product, this is therefore the best Picture Style to use.

If you use Faithful Picture Style in lighting that is not 5200K you will find the
colours in your images can shift slightly and look unnatural.

In this case, you should use Neutral Picture Style instead.
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