Last week there was a discussion about the heavily yellowed Old Timer on eBay. Of course it’s heavily, and quite obviously yellowed - the fact that it was yellowed to the point of looking like a chestnut did not change the fact that it was, and still is, an Alabaster.
It’s also another case where a little knowledge leads to a lot of trouble: in this case, it’s the existence of at least one genuine chestnut Old Timer. A picture of one was published in the July/August 1985 issue of Model Horse Gazette, in an article about test colors and special runs by Jill Rademacher (now Guitterez). Even though the pic is in black and white, it does seem to bear some similarity to the heavily yellowed fellow in the eBay auction:

At first glance, a black and white picture in a defunct, 24 year-old hobby magazine may seem like a rather obscure reference to cite. But it’s not: many hobbyists are packrats of the highest order, especially when it comes to old magazines, newsletters and other reference materials.
I’ve seen hobbyists use more obscure, and much less reliable sources to base their buying decisions on. So while the $75 final selling price was disappointing, it was not unexpected. It just takes two hobbyists to bid something up, and there were certainly more than two hobbyists watching that auction.
What complicated the argument over the Rearing Stallion is the fact that Breyer did - and still does - use colored plastic as a deliberate decorative technique. For a brief time in the 1970s, Breyer used unpainted light gray plastic as a base for some of its gray and black painted models, such as the Donkey, Elephant, and the Spanish Fighting Bull (and less successfully on a few non-gray, non-black models, too!)
As far as I can tell, yellow plastic was not one of those deliberate techniques. Not in the 1970s and 1980s, anyway.
What causes yellowing in Breyer horses is actually quite a complex subject - worthy of it’s own post, easily. To keep it short and simple for today, I’ll just state that it is not unusual for a model to yellow out evenly over its entire exposed surface. It’d be far more unusual, and suspicious, to have a model that was unevenly yellowed.