This particular fleece was rather yellow when I took it out of the bag, and I assumed the yellow was old lanolin and would wash out. But when I washed it, the yellow remained.
It's actually a rather nice sunny yellow; the picture below doesn't do it justice. The yellow goes about two-thirds deep in the fleece, with the last third still being white. I assume that when I spin it, I'll get a yellow-and-white heathery effect. It's too evenly distributed to be sheep's urine, unless the sheep were extremely assiduous about spreading it -- also, the fleece isn't that stinky.
Any idea what could have caused it? And is it anything I need to worry about for the long term?