I have not seen "pure white" Alps key caps, all of mine have been what Unicomp calls "pearl" .....
The PBT caps on old Apples are nice, but the legends are very "different" and you would need the complete consistent set. (I have bought a couple of the horrible "Apple Design" keyboards with nice Alps compatible caps, but I understand that they are not all like that)
Dell AT101s have an ABS, IBM ANSI-standard color set, that are OK but not great.
Others like Northgates and Focus have some really nice doubleshots but are not all ANSI-standard.