Hello, I just bought a H-16 and it needs some work on the hulls.
My question is on which gelcoat should I use on the sides of the hulls, because the sides are not as white as the top of the hulls.
Thanks.
You can get a gelcoat repair kit from West Marine that includes various dyes. That should let you match the color no matter what. 
There are some older hobie hulls where deck and hull has gel coat in different colors... (Jim has one of those - yellow / red). I have never seen a Hobie with different shades of white. Hmmm, here are two theories... Your hulls might be painted while your decks might still show the original gel coat. Or, you hobie spend many years in the water, which might caused discoleration. Check here for some ideas... http:/
Pretty cool to see that my web page is getting some publicity. Before I repainted my hulls, the deck seemed to be a lighter shade of white than the hulls, probably just from being in the sun. You could also see the difference in shade, and even thickness, when I removed the pin stripes.
I saw your posting about the staining that naturally happens over time to the hulls. I have had amazing good luck with a product that make clean up easy. Take a look at "FSR" by Davis Instruments. I have an babied 84Hobie16 with special edition blue hulls. My hulls were truly green from all the yellow satining on them over time. FSR pulled the yellow away and the difference was immediate and noticable as I progressed alonog the hulls. I loved that it did not involve buffing or a bunch of elbow grease.
Good luck!
Doug.
(No affiliation to FSR or Davis Instruments)