Colorado tubing is cone shaped, and was made for Serotta out of whatever preferred alloy Columbus, Reynolds, Tange had available at the time.

So it is a shape designed by Serotta and made by a tubing maker to that spec. It is not shaped like other tubing Columbus ever sold. The only thing I've seen like it is the conical swaged tubing Litespeed made into one version of the Basso Gap Ti.