The Microsoft Store is only selling the black version of the Lumia 550. A few weeks ago, B&H Photo began selling the same phone, but in both black and white versions, for $149.99.
Just a reminder: The Lumia 550 has a 4.7-inch, 1280x720 display. Inside, it has a 1.1GHz Snapdragon 210 quad-core processor, 1GB of RAM and 8GB of internal storage, which can expand by up to 200GB of additional space with the microSD card slot. It has a 5MP rear camera, a 2MP front-facing camera and a 2100mAh battery.
Thanks to libra89 for the tip!