Recently picked up a copy of "1999" (Germany, 7599-23720-2) on the basis of the print on the CD being purple instead of black. Was this a limited run?

Unlikely.

It's not uncommon for discs (not just Prince) from different countries to contain different text, colours etc.

Yeah my cd copy was purple too

They're all printed in purple in most (if not all) European countries. So no, it's not limited.

They used to be printed in black back in the 80s. However, at this time D.M.S.R. was missing off the tracklist and the tracklisting on the back of the case was in red lettering on a purple background.

Then later on (in the early 90s I think), D.M.S.R. was added back onto the album and the CD print color was changed to purple as well as the back of the CD case being changed to yellow writing on a purple background. The color difference became an easy way to tell the original 10 track and new 11 track relases apart.

But I have a 11 track Germany copy of the CD with black text.

Mong said:

But I have a 11 track Germany copy of the CD with black text.

No, that cant be! Don't you know that "fishy Boris" is always right!