You can now buy the OnePlus 6T in Thunder Purple
https://www.cnet.com/news/you-can-now-buy-the-oneplus-6t-in-thunder-purple/
The Thunder Purple OnePlus 6T costs the same as the other colors, which is $579/£529 (about AU$817 converted) for the 8GB RAM/ 128GB of storage option. As of right now you can only get the purple phone in that configuration.
The OnePlus 6T brings fast performance and features like an in-display fingerprint sensor and tiny notch. It costs much less than other flagship phones on the market, which makes it an appealing option. You can read CNET's full OnePlus 6T review here.