The various " BN " Casade greens from different sources result from
the use of different pigments being used by different manufacturers/
suppliers . The weathering to a " bluer" shade is indeed caused by
the " loss " of yellow in the dried paint film . This loss of
yellow is due in large part to the fact that most bright yellow
pigments are organic compounds and are not very light stable . Blue
pigments ( especially - the cheaper iron blue types don't do this as
badly ) are more stable . If you were to look at a good
automotive grade " green " paint formula , you would find it is
formulated with " green " pigment not the combination of blue and
yellow to give a " green " shade.
The vehicle type also plays a role in the weathering of the
paint films .
If you had a shop who bought alkyd based enamel color , the film
will " yellow " for awhile until it starts to chalk - then it will
start to " whiten " .
Toward the end of normal " BN " freight car painting a water
borne type coating would have been used that could weather " brown
" and then fade to the normal " whiter " shade .
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