It's all about the weight, Philip. Special paint schemes are painted in layers, such as this photo of the current QF wunula before colours were applied:
http://www.airliners.net/photo/Qanta...-ER/0381828/L/
your standard aircraft will have a base colour with titled applied over it, but the special schemes have a considerable amound of paint done over the primer, hence more weight is added.
from answers.yahoo.com these figures are shown:
747-400 fully painted-555lbs
747-400 polished with painted customer markings-55lbs
but that is for a 'standard' eurowhite aircraft, so you'll understand that it would be a greater difference for special paint schemes!
For all your QF drivers out there, are the performance figures considerably difference for VH-OEJ due to the paint?
Nick