Seal is getting dropped from the color palette. The only way to get it would be as paint to sample (not to be mistaken with "special" colors like GT Silver), which means a lot of money and a much longer wait.
It's getting replaced by Meteor, which is halfway between Seal and Slate.

I don't think it'll have any impact on the resale value at all. Seal looked great and as far as I can tell, so does Meteor. Most potential buyers won't even be aware of the change, they're just after a grey Cayman after all.