What an interesting point.
Isn't red glass expensive and hard to handle?
One reason for finding a lot of various colours in this design is its long production period. In the -65 and -67 export catalogues there were less colour options than in those of the 70s. It seems the red items are found in items produced from 1970 on.
It has been documented that "In 1965 the export catalogue gave the following alternatives: clear, brown, lilac, blue, violet, blue-green and blue-grey"