It's unlikely to effect the colour.
Several discussion about this kit on the forum, but they didn't mention it being darker than expected, but maybe someone who has made the kit will be along soon.
I think someone also mentioned a different kit that had similar problems about a month ago & consensus was that the picture was much lighter Than the actual product.
The only thing I can say are:
If it tastes good, it's probably ok.
Who knows what lighting was used by a marketing department to make the picture on the box.
The cans of extract do get darker over time, but I'm talking about years here - where people have tried brewing old kits they found at the back of the cupboard.
Or if you go "off-piste" and boil up the kit with extra sugar and hops it can get colour through caramelization.
