It's probably because of the pigmentation that is in the feathers of the green budgies which is not found in the blue budgies... It is possible though that a yellowface blue budgie MIGHT have a similar affect, though I wouldn't know as I don't have a yellowface blue... such as one that looks like this...
http://www.merribuck.com/JoeBlue2.jpg
I guess you could say that it's something like the difference between glow in the dark stickers, and normal stickers. Some chemical or something is used on the glow in the dark stickers to make them appear visible in the darkness... and it's because of this chemical that stores light that after the light is turned off, it emits it... And is not found in the normal stickers... I guess this is the only way I know how to describe it best... less I over-read something...