This could be a way to test different marketing strategies -- black phones like Apple's Jet Black iPhone 7, Nokia's limited edition "Arte Black" model and others are in vogue right now. At the same time, it's a way for OnePlus (and Colette) to grab some easy publicity for the 3T model, released in November last year, until the rumored OnePlus 5 comes along.
Despite the tie-in with Colette, which sells a lot of very expensive things, the branded OnePlus 3T will go for the same price as the regular 3T, €479 (around $515). Assuming you could make it to the Paris store on March 21st at 11AM, you'd be able to get a 128GB OnePlus 3T in all-black engraved with the Colette logo and a special box. Other specs, like the Snapdragon 821 CPU, 6GB of RAM, fast-charging and dual 16-megapixel cameras, remain the same.
If France has the same number of rabid OnePlus fans as elsewhere, they might go fast. If you can't make it yourself, we'll try to get over there and have a look.