When the Apple Watch was announced, Apple sold it as a fashionable object as much as a functional one. It's a sensible approach — to be successful it would have to replace the wristwatches that people wear as everyday accessories. It had to compete with watches that were as much jewellery as timepieces. I didn't really care, though.
I picked the space grey aluminium Apple Watch with the black sports band because it was the least expensive and most understated model on offer. I don't wear jewellery, and I didn't want to feel that it would be inappropriate in dressy environments, and the grey/black combo is neat enough and subtle enough to just blend in. The Apple watch is still about fitness and notifications for me above anything else. It's a computer; a tool with a job to perform.
Now I own three watch straps (or bands, as Apple call them), however. Clearly my opinion has changed at least a little.
It started with the Leather Loop, also in black. This is one of the few watch bands that doesn't have exposed steel hardware, so doesn't look out of place attached to the aluminium Watch. It looks smarter than the Sports Band, which was the point. Although I think you'd get away with the black Sports Band, the Loop stands up to a little more scrutiny if you're wearing a suit.
And since then I picked up the bright orange sports band (and matching iPhone case) — for no other reason than I like the colour. I tend not to wear bright colours at all, so it's been kinda fun to wear. It's definitely not going to look good with formal dress though.
So it has become an object of personal style, even if it's fairly subtle or minor.