The US Treasury Dept. has chosen Flex Products Inc., a subsidiary of Optical Coating Laboratory Inc., to supply its patented light-interference pigment as an anticounterfeiting measure on the US $20 bank note. The Santa Rosa, Calif., company designed the pigment to shift colors from green to black when viewed at different angles -- a feature that is extremely difficult to replicate. The $100 and $50 bills already have the color-shifting ink. The patented technique generates an optical phenomenon that is found in the colors of a hummingbird's feathers and in a butterfly's wings. The ink will be located underneath the numeral 20 in the lower right-hand corner of each bill. The US Bureau of Engraving and Printing estimates it will print more than 2 billion of the new $20 notes before its release to the public in the fall.