Yes, we mix all of ours. Last trip I had Blue/Purple combinations. DH had Red/Gray combinations. DS5 had Green/Gray combinations, and DS1 had Blue/Yellow. We plan on doing it again for our May trip with more color combinations.
I really like the combination of grey with other colors, especially pink/grey and purple/grey My friends and I decorated ours with Disney temporary tattoos for our last trip (with some acrylic spray on top).
As soon as you add a band to MDE it is activated. You can have multiple bands activated so you can switch them out any time. You need to actually go into your MDE account to deactivate the bands and most people don't do this unless they have a ton of bands.
I just got one of those for my April trip and I can’t wait to try it out. I always wear my Apple Watch and having my magic band on the other hand was a pain for me last trip. I think I’m going to get a cheap Apple Watch band and decorate it with some disney nail art.