I would also love this feature, there is a work around for you to install "any" app for any other OEM market place, however when I tried installing the HTC app that includes the clock tile, it failed because the Nokia 920's resolution isn't supported .
Also because live tiles are limited to being updated only every 20 minutes maximum, only hardware manufacturers such as HTC, Samsung or Nokia have the greater access to the OS to make this possible.
So in brief, if this is a feature you'd truly love, just let Nokia know as I did. The more people to request a feature, the sooner it will come!
There's one app in the market called TimeMe, it's free but the only inconvenient is that the tile only updates every 20 minutes, however if you have a developer unlocked device, there's a sideload version that you can use.
I currently use this app it has color configuration. TimeMe | Windows Phone Apps+Games Store (United States)
I have 8x. I used the clock tile cause it really brings nice design to start screen. But more you use that app, the more you realize how waste of space it is.