Yellow hay may mean it’s been left out in the field for a prolonged time after the harvest. That isn’t a bad thing, hay that’s been left out in the sun contains more vitamin D than greener hay that’s been collected rather soon after harvest.
A friend of mine who keeps and trains race-horses explained this to me recently. She is super careful when it comes to hay because race-horses are sensitive and prone to infections because they are ridden so hard during race-season. (Goodness, I realize don’t know any English horse-racing-terms, hope you can understand what I’m trying to explain =) Race-horses always gets various types of coughs and injuries because their immune-system is under pressure from hard training. Just like professional human athletes, they have to keep a very nutritous and good diet.)
I can’t tell you what type of hay you have got, but the yellowishness doesn’t make it bad. It could be meadow-hay perhaps. Meadow is very common. If it smells mouldy you should throw it out.