The Sovereign deflector dish spews out a golden light. The other alternate costume pieces for the hull have deflectors with blue light.
Basically when you mix and match the Sovereign hull with other non-Sovereign saucers you end up with lighting that suddenly turns blue on the saucer.
Ew!
Here's an example:
http://img.photobucket.com/albums/v58/PlayBoxCube/goldblue.jpg
And no, there's no logical reason for the yellow light to suddenly become blue. :rolleyes: