yellow stripe is a fairly harmless condition from being "out" in less than ideal conditions in field hay (western guys sometimes call it tiger stripes as well) it is the same sun bleaching that happens when hay (alfalfa or timothy)is stacked in the sun and goes bright yellow....it's a no biggie
frost is really frozen dew and that will leave the same visible damage as dew which is tiny black spots up the stems to the leaves/heads depending on the grass on a yellowed body....for most practical applications on horses you will not see a great difference in the end values....
He says it was exposed to a hard frost after being cut but before being baled. The grass parts of it are discoloured and look like they were rained on but they were not. He does not think it would affect it from a nutrition stand point.
This guy has always been very good to deal with in the past and is very straight forward about his hay so I am inclined to believe him.
Man, I hope our wet cycle is over and we get back to more normal weather next year!