Chris,
Most windows in UK caravans are manufactured by a company called Polyplastic. The sole UK agent for Polyplastic in the UK is Miriad Products. There is a page on their website which describes the change in "shade" which all Polyplastic windows are subject to over time due to UV.
http://www.miriad-products.com/colouracrylic.htm
Replacement windows supplied by Miriad are stored in restricted UV conditions meaning that the shade will be similar to the original colour of the windows on your caravan when it was new.
It really depends on whether you feel your replacement window is a different shade or whether the tint is wrong. If the tint is correct but the shade is wrong then this can be expected. The shade of your replacement window will change over time (I wouldn't like to say how long) to match your existing windows. If the tint is wrong then you should go straight back to the dealer. If the replacement window has been manufactured by Polyplastic then you can check the code etched on the window to see if matches the originals on your caravan.
If your van is under warranty then you may be able to get the dealer to replace any adjacent windows if you are not happy with the difference in shade.
I replaced a Polyplastic window on my previous caravan outside of warranty and purchased the replacement directly from Miriad products. I part exchanged the van 6 months later and the shade had not changed significantly.
Hope this helps. Good luck with the dealer.
Mark.