Does Granger make zir choices for moral reasons or for practical ones? When do those overlap, and when do they differ?

Time-Twisters are normally yellow, the color of the band from this page where Voldemort activates one. They're only blue when they're active, which they have been every time we've seen one.

The desk works like Moody's trunk from the books: If you open it with Granger's key, you'll find the drawer with the Time-Twister in it, but if you open it with a different key, you'll find something different. If you open it with the kind of lockpicks you can get from Weasleys' Wizard Wheezes, you'll find a high-tech burglar alarm system. Tonks personally disguised zemself as a Hogwarts student (Fred and George don't usually sell to DMLE agents) and bought a set, it's in order to make sure DMLE security is safe against the current tools available.