Scott's helmet was also red...We're in march 69 and....
At the time, publications showed drawings of the moonsuit just like Apollo 9, but in black and white.
The white cap was added between Apollo 9 and 11, perhaps because they were concerned by temperatures on the Moon, the LEVA.
The spacewalk's purpose was to test the PLSS, but the most visible change was the helmet...Things changed a lot and fast at the time.
Why it was red? to distinguish the astronaut? I don't really know (never saw any explanation for it).