Hi Rob,
I'm in the same boat as you - got an SDS lid (which I've just finished modding) and was thinking of getting an FX/AM kit to go with it.
I was advised by one of the armourers on the UKG boards that the AM tends to have a slightly yellowy tinge, whereas, as mentioned, SDS is glossy white. He pointed out, as Mark said above, that the film lids weren't perfect matches for the armour anyway, but his opinion (which I agreed with, when I thought about it) is that, as a perfectionist, them not matching would drive him crazy