Yep its down to the different materials and it is made worse by the fact that the bumpers/ car get sprayed with different batches of paint. It also gets worse as the car ages too because paint fades faster on metal surafces than plastic so your bumpers will appear darker with time.
I had this problem with my Alfa Mito, when I went to collect it after waiting 8 long months, I noticed it straight away and I refused to accept it until they sorted it out. After another week I went back and it had been sorted out. Funnily enough though they never painted it, they had another bumper plucked off the production line in Italy and shipped over. That bumper matched like a dream though, so again highlighting the different paint batches problems.
The "higher" end manufacturers compensate by spraying the plastic panels with a different shade of colour ( slight lighter) so that they match perfectly with the metal panels, I know this is the case at Jaguar at least, because I have seen it in action ;)
 
Well it will be ideal if they do sort it out as it is annoying that people say did you know your bumper is a slightly different colour but like seat are known for doing there hardest to fob you off they will probably say there's nothing wrong with it at the end of the day if the car is in warranty then they should just get it sorted to keep a good reputation.
 
Well after a month of arguments with Seat customer service I got a professional paint match report done on the car by someone I know which showed that the back bumper was completely out of colour compared to the rest of the car which left them with nothing to say and have now got the car booked in to have it re sprayed at the cost of the dealer. :D
 
Yea tell me about it I had at least 15 people notice the colour difference on mine and it was driving me mad lets hope the re spray sorts the problem out I won't be driving it off the forecourt if its not right. :)
 
Yea and if you take it to the dealer they will say its because its plastic the friend of mine who got a paint matching machine on my car yesterday works at a company that sprays 3000 plastic parts a day for the likes of Jaguar Ford Land Rover and said that what the dealers say is complete BS and no matter what if the parts are prepted and primed properly then they should match the metal work.