Designers Zac Posen and Thom Browne have just joined a select high-fashion coterie that includes legends like Bill Blass, Emilio Pucci, and Hubert de Givenchy. Membership is based not on an exclusive Paris runway show locale or the styling of an outlandish couture gown, but on the fact that they have all designed a special edition of a car.

Browne and Posen have just teamed up with upscale online shopping hub Gilt.com to each create a bespoke one-off vehicle. Both of their cars are based on the new Infiniti Q50 sports sedan, and each will go on sale on December 12 for $75,000. (Included in that price is a $10,000 Gilt.com shopping credit.) While the cars are identical under the skin, their appearance couldn't be more different.

Browne's version has a striking mirror finish over its entire surface, like a giant pair of highway patrolman's sunglasses. "Gilt said that they wanted the vehicle to be a reflection of their customers," Browne told me at the car's New York unveiling. "So it's sort of a play on words." Inside, the seats are done up in black pebble-finished leather and inlaid with Browne's iconic red, white, and blue piping; a matched set of hard-case luggage, in precisely the same livery and materials, fills the ample trunk, which is also rimmed in patriotic bunting. "My signature is in the interior," Browne said. "You open the car's doors, and you see me."

Posen's version reflects what he called, "red carpet-ready glamour." His car's exterior features 36 coats of gray paint, fading, like the trendy ombre hairstyle, from a flat matte up front to a pearlescent high gloss in the rear. His interior design is also keyed to his personal aesthetic, coated in deep burgundy leather with red velvet piping, and richly textured, almost glittery shagreen (stingray hide) covering the armrests, center console bin, and steering wheel rim. "It's meant to be mysterious," Posen told us, "moving from the shadows."

Both designers were pleased to participate in this project in which a portion of the proceeds benefits the St. Jude's Children's Research Hospital. Both were open to future automotive collaborations (if the correct partners were found). And both emphasized the unique nature of their potential buyer. "I expect them to be one of a kind, and very chic," Posen said. Browne agreed. "The car is for someone who likes to make an entrance."