The exact color of the Ray-Ban sunglasses that Brad Pitt is wearing is Silver frame, Grey gradient lenses, color-code 003/8G. Unfortunately these extremely cool sunglasses are discontinued by the manufacturer and very hard to find. A similar pair are the Ray-Ban 3341 which is still available for appr. $129.00, visit Amazon.com for information and images.
Reference: Ray-Ban RB 3291 - Color: Silver frame, Grey gradient lenses (code 003/8G)