This app was mentioned in 3 comments, with an average of 1.33 upvotes
If you're on Android, I built an app that does such things. You can find it in the playstore here https://play.google.com/store/apps/details?id=org.hackerforhire.msbandlight
I just saw your comment and took the few minutes to update mine with color picking (for now the default MS colors because that was quicker). If there's interest in the future, I'll see about updating to allow the whole color pallet. ~~Give it a few hours and~~ The update should be live (v2.2) and it will allow you to select colors other than a full white band.
So I tried going down this path ... I found out a lot of interesting limitations to the band. I've got a "flash-light" app ... but there's things as a developer I can't control:
ability to put custom background / color to notifications ... you only get the side bar
trying to solve it by using ascii 馃敠 characters, but the character set only support showing the hollow block, not the full one
Either way, it was a good learning experience and ~~I'll toss it up in the Android store for anyone that wants it~~. That didn't take long. Still not as powerful as a phone's LED's though :( oh well, maybe we'll get a real SDK one of these days...
This is available form Android: https://play.google.com/store/apps/details?id=org.hackerforhire.msbandlight