from the things i read one of the dangerous colors was yellow, but they found that red hulls were attacked as well and there are many reports on white and off white, a lot of people points out that contrasting colors attract their attention, some divers in completely black wetsuits report not being bugged while others in more colorful ones were, that would throw off the prey thing because i never seen a seal with colorful strips and for sure a guy in a black wetsuit but look awful like one. Seems that the zebra stripes are a good solutions for keeping them away, some guys have a business selling those decals for surfers and kayakers.
Beside that you have to think that even if they now have proof that sharks identify their prey by sight too, and they can see really well, an hull seen from underneath look like a dark silhouette in most cases whichever color it is.
Seems that knocking off people on kayaks its an habit that they have, probably plain and simply some sharks that are used to see kayaks and people around know that its a person on top of a kayak and that one of the two its not good food.
Seen it happening with other animals, if you paraglide close to an eagle nest the eagle will go at you not at the glider or the harness. Some animals are smarter than what we would think and shown to get how things work even if they never seen it before and put together constructive plans, and with something new thats not just instinct.