If you head over to Nokia's Facebook Page, you get a chance to demo different colors of any of the Lumia phones...
You can demo the Lumia 800 in black, white, magenta and cyan. The Lumia 710 with yellow, black, white, cyan and magenta back covers. The Lumia 610 in white, black, cyan and magenta also.
The Lumia 900 is currently available in White, Black and Cyan, no word from either Nokia or At&t if the phone will be available in Magenta, which seems to be the only color missing from the Lumia line-up. But it is now on the Nokia Facebook "Demo the Lumia" Facebook page.
Well up until now, the magenta version was only spotted on the demo screen at at an At&t store by a WPCentral reader...you can see that photo here
Rumor has it that this version will be released on Mother's Day here in the US.
This sighting only proves that this version does indeed exist, though there is no word from Nokia or At&t camp on when it will hit stores.