Not a dumb question at all. I don't personally know of any law requiring yellow cans for diesel, although some states may have such without my knowledge. When I was using a 5 gallon container, I was using a red one because that was what I had and I hadn't seen any yellow ones in the local stores at that time. The red one I used was a different brand and shape from the 5 gallon red can I used for gasoline, and I labelled the diesel can in big letters on both sides. I would still prefer yellow and if I were buying a new can, I'd look for a yellow one. No one ever questioned my use of a red can, but of course, I bought my fuel then at self serve places. Then when I went to a 55 gallon drum, it was blue because that was what I got free. And I had that filled at a wholesale fuel distributor and no one there ever questioned the color.