Just a thought... purple and red are among the least colorfast of colors. It will be several degrees hotter down below, in a tropical summer, as well as fade MUCH quicker than other colors.
About your other qusetion... two part LP paints, (like AwlGrip), are not meant for full time immersion either. If you put a boat cushion on a LP painted deck
, and keep water
against the paint for say... 6 weeks, it will blister! Same if you went around the world on the same tack. This is not likely, but after long enough "wet time", LPs blister. Many world cruisers never notice the little blisters
, because either it was not long and consistent enough, or after a month on the hook in Tahiti
, the blisters
"went away". The adhesion never comes back, but they do go back down.
If the automotive paint is like two part acrylic
, "PPG paint", it should hold up on your hull fine, and is a second runner up to the champ... AwlGrip. It just wont be as durable or shiny.
BTW... If you prefer AwlGrip, and are willing to experiment
, (=$$$), you can mix your own colors. I made a light green by mixing some well shaken green base, with well shaken white base. The ratio of catalyst to base is the same, no matter how many colors of base went into the mix. So when the base is your chosen color, mix in the appropriate amount of catalyst, solvent, drops of accelerator, and you are ready to go!
PS... When I changed our cabin side stripe from a broad expanse of green, to just a contrasting border stripe of green, the effect was still there, but it was much cooler below in the summer. (And ALWAYS use gray primer, as it is opaque, and blocks the UVs off of the hull better.) This is cooler yet, and easier on the plastic hull surface it covers.