What is the difference between blue and silver seal on the five dollar bill?

$5 bills have been printed with red, blue, and green seals.

Blue = silver certificates. These were redeemable for $5 worth of silver up till the 1960s.

Red = U.S. Notes. This currency was issued at the national level, again up till the 1960s.

Green = Federal Reserve Notes. The only currency now issued, it is released through the individual Federal Reserve districts.