You know what happens if you leave a book spine, or even a decal sheet, exposed to bright sunlight - extreme fading.
Point taken, although that wasn’t exactly the OPs question …
Says who?
The question of faded colours is perfectly relevant to the OP's question.
It is sensible to draw the conclusion that before the advent of protective coatings PC10/12 and stronger clear varnishes, less attention was paid to the effect of strong sunlight on fabric and early war machines CDL and national markings would be lighter after a short time in the field.
I attach a picture of a pair of my shorts, worn from new, on the Loire for a week earlier this year.
Under the fold the back panel is the original mid blue, the front panel shows the effect of the sun, exactly as I believe would have happened a century ago.
I'll always build early CDL RFC aircraft with lighter roundels for this reason.
Please don't tell me I am wrong Mr Colour Policeman - unless you are prepared to show us the colour of your pants too!

Sandy