Hi Dave , its gotta be Orange Tip, it should as you know quickly turn orange in colour. You only need one passing female to disperse eggs on suitable plants. I have been tracking down Orange Tip eggs in my area for about 4 weeks now, and they do turn up in areas where no breeding colonies exist.......the females travel long distance seeking out Cuckoo Flower or Garlic Mustard. I was out walking on the moors where I live ,and came across 1 solitary Cuckoo flower in the middle of nowhere, and lo and behold it held 1 solitary OT egg.......!