Who says it's car paint though? White oil based enamel paint and the like is notorious for yellowing with age. (Radiators, old house gloss, etc)
I discount all pictures of the roundels in daylight because it will always look bright and white that way. So we need to look at the roundels in the gloom and the best we can manage there is in the boathouse in 66/67-
In the pic above, it looks as if a flash gun has been fired and the front of the boat is a little washed out as a result; further back, along the boat and into the shed, the sponson darkens somewhat and yet the roundel is still a crisp white.
In the pic below, all the colours appear quite over saturated but still the roundel is white (with perhaps a hint of a warm cast that is over the whole picture)
But, but but but...the roundel is now a very definite sort of creamy-yellowy colour. I further polished a little area on it and it was coming up whiter if you squint, but like old house paint, it's pretty uniformly discoloured.
Then there's the other white paint on the boat- the lettering on top of the sponsons and the flags- all crisp white. Why do the roundels differently?
I think they should be white, and the original has gone off with age.