In addition to a different "Holiday Bronze" color for 1956, ’57 and ’58, there was a tremendous range within each year. The process was not as ridgidly controlled back then as now and there were big variations. This could depend on how often the pot was mixed, who did the painting and it’s even possible they were mixed by eye!
I have a bunch of ’57 and ’58 Johnson motors and (within each year) every one of them is a slightly different color. One factor is that the cowls were typically painted the accent color (white) first – so, depending on the paint thickness, it can make the Holiday Bronze on the cover a lot lighter than the block and leg. Also, 57-8 years of oxidization can make a difference too! I emailed with Peter McDowell about this years ago. He tried to find the best examples of each year and mixed the paint to match – but he conceeded that the range was vast and he could only do so much….
Here’s my "closet" AD that was reportedly used once and then spent almost 50 years unused and in a bedroom closet. There is no doubt this is original, however, the color on this is a lot more red than many other ’57s I’ve owned or seen. If you look closely in the photo below you can see the color difference between the hood and leg – this was due to the white accent color having been painted first under the Holiday Bronze:
