Hi Nik, Ruard,
Here's my interpretation of the pattern:
Hook: Standard wet fly
Thread: Yellow
Hackle: Golden Hackle from a Ring-Neck cock (on neck) or Red Cock
Abdomen: Yellow silk floss
Thorax: Peacock herl
Head: In front of hackle-Peacock herl
The reason for the discrepancy could be the printing process for one. Often times, what is printed in an illustration may not be color corrected to the actual illustration. Back then, I'm not sure how much color correcting they did. Over the years, the original illustrations may have gotten lost, and the only prints they have are of the uncorrected kind. Now all prints of those plates show an off color.
The only other explanation might be the body was waxed, and this darkened the thread, somewhat, making it more an orange, but the instructions do not mention waxing the thread, so I have a feeling it's more the first reason.
Mark