The seal colors for US currency have changed over the years.
Yellow were for gold certificates, except the ones made for North Africa during World War II.
Blue were for silver certificates.
These no longer can be exchanged for gold or silver.
Brown was for the Hawaiian Islands during WWII.
The North African and Hawaiian currency were made with different colors so they could be demonetized in case of invasion overrun.
Red were on the two and five dollars.
Green was started when the Federal Reserve started (I think).

The note above the $5 bill is from Romania, its made of polymer, and celebrated the year 2000 and space.