Shouldn't dull if its been lacquered. Like mentioned painting plastic always turns out a slightly different shade but shouldn't be too noticable. Only other things it might be is not enough paint / too much lacquer or a different paint codes been used. Was it a good match when you collected it?
I only collected it yesterday and it looked spot on in bright sunshine but today its cloudy and the spoiler looks more of a brighter white. Should I go back to seat you think ?