I had black for 9 years on my old 390EC............don't do it. We are in Florida and with an aft enclosure, the heat in the summer was almost unbearable. I have linen tweed on the 450 and there is a noticable difference in how hot the cockpit is in the summer.
The other problem with black is that I found that after about year 3, the canvas began to look washed out and a little faded..........it just showed its age sooner than lighter colors.
FInally, any time a vendor tells me something like its not available, my b.s detector begins to spin. Go to this site:
http://www.sunbrella.com/na/en/marine.pl
Select fabric showroom. I found Turquoise, #4610; Aquamarine, #4623; Persian Green, #4643; and Mineral Blue, #4687. Only you can tell if one of those matches your boat and suits you, but my view is that any one of them is better than black, which, aside from the heat and colorfast issues, just isn't going to look right on your boat.
You are dealing with an OEM vendor and it lis likely that what is going with the "its not available" is that Great Lakes meant "we don't stock that any more".