Whew! You had me worried there for a minute. I’m happy to hear that your seahorses are courting and not ailing.
Yes, the change in coloration you describe sounds very much like the typical courtship colors is played by Mustangs and Sunbursts (Hippocampus erectus). Courtship coloration varies from species to species. However, regardless of the colors involved, the head, dorsal surface (i.e., back), and ventral line (keel) of the seahorse normally remain quite dark while the rest of the body becomes lighter and dramatically intensifies in color (Vincent, 1990). In H. erectus, the brightening that occurs during courtship usually involves the body of the seahorse turning a silvery white or cream color, while the head and dorsal area remain almost black.
As long as your water quality remains good, with no ammonia or nitrite spikes, and the seahorses are displaying a healthy interest in courtship or breeding, it sounds like you’re new set up is on the right track, Nigel. Here’s hoping the courtship is successful and continues to its logical conclusion so that your pair of seahorses provide you with a brood of young before long, sir.