Bob,
I have seen photos of the cars on the WP&Y about 1957 I think, and the cars were a very bright yellow then. I know that the cars weren't green at the end of SV operation though, pictures from 1947 clearly show them as a very light color, either pink or yellow. I do have a photo from the Alder Springs wreck in 1927 and it has a good close up of coach #26. The paint is very dark, so I would say that the cars arrived green, were painted pink, and then later painted yellow by the WP&Y, probably in the 50's.
Thanks Again, Taylor