Normally if it is still within the 14 day remorse period you can return for refund and buy another in a different colour I guess. If it's outside that period I don't think they will just exchange unless it's faulty and it will be a unit of the same colour.
However it's xmas so this help -
"Holiday Return Policy
Items purchased at the Apple Online Store that are received between November 1, 2013 and December 25, 2013, may be returned through January 7, 2014. Please note that all other terms and conditions provided in the Apple Online Store Sales and Refunds Policy are still applicable with respect to such items purchased. All purchases made after December 25, 2013 are subject to the Standard Return Policy.
Standard Return Policy
Returns can be arranged via our online Order Status website in one single step.
Returns are subject to Apple's Sales and Refunds Policy. If you change your mind, you can return your Standard or Configured-to-Order hardware to us within 14 calendar days of delivery. Software returns are also accepted within this period, but only if the software was not unsealed. This option is in addition to your consumer rights to return defective products."