Can I buy a att Lumia 920 or 8x and use it for tmobile with an unlock?

I want to buy an 8x but my bro already has the blue one they offer and i don't want to get the same color as him, can i buy a lumia 920 or the yellow htc 8x from att and unlock it to use it with tmobile?
if so, would there be any restrictions? Would i still be able to get 4g?

the red 8x from verizon is probably my favorite color, is there any way i can get that one on tmobile? I know they're different networks, i'm just curious tho haha

please no iphone or android comments, i like WP and i don't plan on going back to android or iOS anytime soon

You can get either the Lumia 920 or 8X from AT&T and, once you get them unlocked, you can use them on T-Mobile, albeit you will only get data with 2G speeds because T-Mobile uses 1700 MHz vs AT&T's 1900 MHz for data, except in areas where T-Mobile has refarmed their towers to use 1900 MHz (see link in source for areas that have been refarmed).

However, can also get the 8X from Verizon. Not only does it come unlocked for AT&T and T-Mobile, it has been confirmed to work on both carriers by Engadget. See link in source below for more info. Again you will get data at the slower 2G speeds with T-Mo unless you're in an area that has been refarmed to 1900 MHz.