OK, these are the photos:

My guess is either the hull was resting on something for a long time and the light changed the colour where it shone on the kayak (this has happened to me, but you need to be building somewhere with quite a lot of light, and be VERY S L O W :-) or something was spilt on the hull and wiped off, but left some difference which shows under epoxy. Either way, I would just leave it.

For the deck, it really is worth wetting down before the final sanding, as it shows any glue lines, contaminants etc, and also raises the wood fibres meaning a smoother finish after the final sanding