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 ?
 
It really depends on how happy you are with the colour match, could you live with it? If not then take it back to get checked.
 
It isn't bad but I personally couldn't live with it I would rather get the whole roof and spoiler done gloss black than have a colour mismatch. I think they should re-do it tbh because I've had bits of plastic trim sprayed on my old car before by a little back street garage and they got it right so I didn't think seat would get it wrong tbh
 
It really depends on how happy you are with the colour match, could you live with it? If not then take it back to get checked.

Hey last time i hijack this thread now :p took the car to seat and they accepted that there was a problem and are fixing it friday cheers for the advice.