A Hong Kong A600 I have is very white indeed; as white as any other white computer I have ever seen. A UK made one I have isn't as white (plastic on the inside is not as white either). Whether plastic can go a bit yellow/off white without light contact I'm not sure, but maybe it used to be as white as my other at one time. Amiga's were made in the Philippines and Singapore as well, so there's plenty of opportunity for different case plastics.
Check this out from the Gallery:
http://www.amiga.org/gallery/index.php?n=3756Note that the A1200 is as white as those A600's. Note the beige A500.
I thought Escom A1200's used cases manufactured by Commodore (old stock) with just new badges?