Wasn't sure where to post this but since the FR235 has the optical HRM that requires a tight band it's probably the most applicable here.
Looking at this page https://buy.garmin.com/en-US/US/shop-by-accessories/bands/watch-bands-forerunner-230-235-630-/prod532676.html all bands appear to be interchangeable and they certainly are but having just upgraded to the Black/Gray FR235 from the Black/White FR230 I was surprised to find that the Black/Gray strap is a lot softer/thinner/flexible than the Black/White strap. I'm guessing this is to allow the FR235 to be worn more tightly without being (too) uncomfortable, anyone else notice that?
Something to keep in mind if ordering a second/replacement band.
The first three (Black and White, Force Yellow, Purple Strike) being the original FR230 bands, the next 3 (Black and Gray, Marsala, Frost Blue) for the FR235, the next 2 (Black and White, Midnight Blue) for the FR630 with a slightly different design and the last one (Black and Force Yellow) for the Nike Black and Volt FR 235 (see https://forums.garmin.com/showthread.php?334738-Garmin-Nike-Black-and-Volt-Colorway-Forerunner-235).