Not if your already on contract with them unless you placed the order for a white or black one then you have to stick with the one you ordered unless you return it under 14 day returns policy as stated by the network by saying you changed your mind you want the other one.
If pay as you go no chance. Sold as seen unless faulty then you would get repaired/replaced but for exact model/gb/colour you had.
Hope this helps.
Sent from my T-Mobile UK iPhone 4S using iPF.net