There is truth to both explanations. They wanted the parks & the park icons (castle) to look great on film. Older technology in cameras meant that there was ALOT more thought that HAD to go into the color scheme of an environment to make photos look 'magical' and like no place else (because this wouldn't normally exist this way). One of the tricks was to choose to use colors that look great on film with the Florida sky as the backdrop. So, then red was a very real option and since they were already using theater techniques and thinking to bring guests 'into' the story/screen, red made more and more sense. So the 'red carpet' was 'rolled out'. It you want to validate this further look into the back story behind the American Adventure. There are MANY shades of white used on this building, so that it would look great on film.
Both stories are right/true, which I think makes that bit of knowledge that much more interesting.