Yes, as others have said, pastel super fire is possible. Because firefly is not the result of a single gene. It is the result of two mutant genes working together, and each gene is in its own gene pair.
One gene pair has a pastel mutant gene and a normal gene. The second gene pair has a fire mutant gene and a normal gene.
By the way, in that WoBP firefly x firefly mating chart above, there are two boxes marked firefly that should have been combined. Also two boxes marked superfly that should have been combined.