You can head over to their website, regardless if you are from the United States or another place, and select the color options and storage of your choice. OnePlus was an exception with the 6T, following the Henry Ford mantra of "any colour, as long as it's black".
At launch, OnePlus was surprisingly quiet when it came to particulars about the T-Mobile version of its Oneplus 6T, but in a post published late last night to the company's forums, additional details about both the partnership between the two companies and the differences in the T-Mobile version of the phone were discussed at more length.
For pricing, you can get it in China now for ¥3,399 (around $490) with 8GB RAM and 256GB storage. Hopefully, the device will appear elsewhere in the world, but if it does, it will likely keep the $579/579 tag like the Mirror Black and Midnight Black 8/128 GB variants. It is worth noting that the OnePlus 6T in China comes with HydrogenOS, not OxygenOS, though it is quite possible that this variant of the phone may become available outside of China in the near future as well, though OnePlus did not share any info regarding that as of yet.