This is a result of storing hay at high moisture content. The proteins in this hay have usually been chemically altered and become unavailable to animals. Yellowing hay, especially if it is grass hay, is an indication that the plant were over-mature when they were cut. Texture or Softness is important because intake will be greater for soft hay as opposed to brittle hay. If hay is brittle it may still have nutritive value but if the animals have trouble eating it then intake will definitely be lower.
Purity is more of a marketing scenario concern.
Hay that brings top dollar on the market is usually pure, but impure hay can sometimes be more beneficial to the animal because of the increased nutrient value, as is the case for immature legumes mixed with mature grass hay. Condition of bale is also another marketing point. If the bale is poorly shaped it will be harder to handle and more apt to break. This would be a concern of those feeding small square bales a few at a time. Poorly shaped bales are harder to store and load for transportation and they increase the chances of loss.