I have often been asked about the different colors used for on the Treasury Seal on US Currency. This is because there were many different types of currency used at one time. Different series overlapped other series in use. Here is the breakdown of dates/types/colors...

The six kinds of currency in 1929, colored coded with the colors of their seals and serial numbers, and with the denominations they were issued in series 1928 and 1929 (though not always in those years themselves), were:
• (RED) United States Notes (Series 1928: $1 $2 $5)
• (ORANGE/GOLD) Gold Certificates (Series 1928: $10 $20 $50 $100 $500 $1000 $5000 $10,000)
• (DARK RED) National Bank Notes (Series 1929: $5 $10 $20 $50 $100)
• (BLUE) Silver Certificates (Series 1928: $1)
• (DARK RED) Federal Reserve Bank Notes (Series 1929: $5 $10 $20 $50 $100)
• (GREEN) Federal Reserve Notes (Series 1928: $5 $10 $20 $50 $100 $500 $1000 $5000 $10,000)

The three kinds of currency that remained after the Depression were:
• (RED) United States Notes (Series 1953: $2 $5)
• (BLUE) Silver Certificates (Series 1953: $5 $10 -- Series 1957: $1)
• (GREEN) Federal Reserve Notes (Series 1950: $5 $10 $20 $50 $100)

And all that remained by 1970 were:
• (GREEN) Federal Reserve Notes (Series 1969: $1 $5 $10 $20 $50 $100 -- Series 1976: $2)