Actually, judging from Google image search for current horseshoe magnets, this red magnet convention still seems to hold. My questions are when and why did it begin?
I believe the ends are left uncoated/unpainted (hence gray/silver), and the rest of the magnet is coated/painted red. .
Yes, I had a horseshoe magnet colored like that as a kid in the 1970s, so they did exist back then.
OK, reminded of this after stumbling across some episodes of the old spy sit-com “Get Smart”; in one of the episodes the villainous group Kaos uses a gigantic red horseshoe magnet to hijack ships. That episode was filmed in the late 1960s. Not sure what color horseshoe magnets the Coyote in the old Road Runner cartoons used…
Probably just tradition now, although one can come up with a logical reason, true or not:
Horseshoe magnets are mostly going to be used to pick up things, rather than caring about which pole is which. You might be using them for that underwater, or in some other low light condition - the red provides visibility.
I prefer to think that red light is the only frequency that can escape a magnetic field 
I remember learning (either from the Straight Dope or the old Imponderables books), that barns were usually painted red because red paint was cheap to make. Perhaps the same reasoning applied to magnets: red looked good, and wouldn’t add much to the cost of the product.
They weren’t. The “working” horseshoe magnets I’ve seen and used were unpainted, and simply the color of the metal. I had a hefty one when I was a kid, and wondered why mine didn’t look like the ones in the cartoons.
The only ones I’ve seen with the distinctive red coloring were demonstration or educational models, usually much weaker than the one I had. I have no idea why they were thus painted. To make them more visible, would be my guess.
Aha - Yes, that is pretty much the style I described in my OP, red with unpainted (metal grey/silver) ends - to me (and I guess many others over the years), the stereotypical horseshoe magnet color scheme.
I realize that industrial magnets are likely to be left in an unpainted state (like the bar magnets, and rare earth magnets I have), but nothing stops even them from being painted red (that one has a keeper bar at the end).
What is the Straight Dope about the keeper bar? Will a magnet gradually “run down” like a battery without it?
I wonder if this was just a usability factor. You use the magnet to pick up bits of scrap metal. What happens when the magnet gets mixed in with your scrap metal? If it it’s unpainted it looks like all the other scrap metal and you have to spend a few minutes searching for it. If it’s painted it’s easy to find among the scrap. And red was just an easy, cheap, durable paint color that would stand out among the scrap.
I’ve always seen them painted red on one end and blue on the other, so you could tell which was which.
Also, the keeper prevents other magnetic materials from being attracted to the magnet.
Right answer, wrong explanation. There is energy in a magnetic field, but if the magnet’s just sitting there, it doesn’t cost any energy to maintain the field.
Energy is released when a magnet is demagnetised. With the keeper there, less energy is released.
Also, some energy is required to demagnetise.
Modern rare-earth magnets are very difficult to demagnetise: the energy required is more than the energy released. High power rare-earth magnets are stored and shipped very carefully, but that is to stop them from sticking to things, or pinching things, or shattering.
Old-fashioned mild-steel magnets were very easy to demagnetise. I think that the energy released by demagnetisation helps pay the energy cost required for demagnetisation. The keeper reduces the energy available for demagnetisation, and thus reduces demagnetisation.
AlNiCo magnets are between “old” mild steel types and “new” rare-earth types. They are normally sold with keepers, and I think (not sure) that the main purpose of the keeper is to prevent the magnet from sticking to things.