Steve,
I'm pretty sure the yellow GT show car is something other than McLaren orange. I know that color pretty well and it is very definitely orange. The new GT looks a bit more butterscotch than the original GT40 yellow, but not as orange as McLaren's color. On the website you list one of the two McLaren F1s looks close, but I think the lighting is making it look less orange than it really is.
www.racingicons.com has a yellow original car shown that they restored. They may give you that color code which is a great yellow, but I think it is a bit brighter than the new car. I wish I could be more help. Good luck.
Scott