there were dozens of “primrose yellows” in use throughout the entire British car industry during the 1960s and 1970s. Even for the same manufacturer there is no guarantee that a “primrose yellow” of 1962 is the same as of 1975. My own SII car wears its original Fern Grey paint. Fern Grey was in use already during SI production - but in a different tone.
Add natural change over the years (aka fading).
Add effects in the course of photography (white balance, light colour … bright daytime or sunset, effects).
Add differences in TFT displays.
At the end of the day the only way to get a halfway decent picture of the original tone is to have an original colour chart (preferably on metal sheets) that has been stored in the dark.
A less than perfect way would be to ask people with original cars of exactly the same color and year to send you neutral pictures of their cars.
Maybe to begin with you might check your original paint code, find out the correct term and then search for matching cars. What year is your car? In 1974 there was an additional colour “Pale Primrose” (just as “Signal Red”) available only for E types (that being said I saw Signal Red on XJ Coupés which I believe to be original, so it might be that “Pale Primrose” found its way on the skin of XJ sedans as well on special order). This “Pale Primrose” is fairly light with much yellow and a hint of green.
75 XJ6L 4.2 auto (UK spec)