We have discussed blue paint shades before, and factors which lead to differences include :
- Deterioration of colour film/prints over 50 years.
- Different compositions of film (particularly Kodachrome compared to the rest)
- Particular propensity of blue pigment in paints to fade (hence why it was less commonly used than red or green)
- How the paint is manufactured/mixed/thinned
- What it is painted on top of (I think this one is key; it explains why so many diesels repainted BR blue took on a greenish tinge)
- How many coats
- Whether varnished
It's not unique to blue. The cream used on rolling stock varied even more widely; it's deterioration and piebald touching-up appearence was key to it being given up.
Separately, how many others here look out for the very nicely maintained AL6 in electric blue, with white cabs, which has been on the Up side at Rugby for several years now. I passed there not long ago on a first post-lockdown WCML trip, and was pleased to see it still there.