[RightWhale]

Might be related to the rise of color vision. Do dogs see in black and white? Dogs wouldn't care about ochre if that is so, but dogs do arrange things according to other attributes.

[CobaltBlue]

My older son did an experiment for science fair that seemed to prove that dogs can distinguish yellow and blue. Unfortunately, it was disqualified because of rules about "experimenting on animals" - in this case, feeding our Golden Retriever, Lizzie, morsels of dog kibble on various colored paper plates. And now he's lost interest, so it remains for later generations of science to research.

[PatrickHenry]

Do dogs see in black and white?

I had a dog who could identify Federal Express trucks, UPS trucks, and of course, mail trucks. Whenever one of them would round the corner, she'd go nuts, expecting that they'd pull into the driveway, as they sometimes did. No other trucks would set her off. I assumed it was their colors that she recognized -- at least UPS, which is somewhat distinctive.