The Federal Reserve Bank of San Francisco has many images of United States currency on its website. Some of them appear below as examples of the variety of color that has appeared on US money.

Large size notes
1861 $5 Demand Note no seal, red serial
1862 $2 Legal Tender Note red seal and serial			obverse: green overprint
1869 $10 Legal Tender Note red seal and serial			obverse: blue and green overprint
1874 $5 National Gold Bank Note red seal, serial and bank number			reverse: California state seal in left oval
1877 $10 National Bank Note red seal, serial and bank number			reverse: Kentucky state seal in left oval
1880 $20 Silver Certificate brown seal, blue serial			obverse: brown "XX"
1880 $2 Legal Tender Note brown seal, red serial
1882 $500 Gold Certificate red seal, blue serial			obverse: gold overprint
1886 $1 Silver Certificate red seal, blue serial
1889 $2 Silver Certificate blue seal and serial			obverse: blue "2"
1890 $1 Treasury Coin Note brown seal, red serial
1891 $5 National Bank Note red seal, serial and bank number			reverse: North Carolina state seal in left oval
1900 $20 National Bank Note blue seal, serial and bank number			reverse: no state seal
1907 $10 Gold Certificate gold seal and serial			obverse: gold "X"
1923 $1 Legal Tender Note red seal and serial			obverse: red "1"
Small size notes
1928 $100 Gold Certificate gold seal and serial
1929 $20 Federal Reserve Bank Note brown seal and serial
1933 $10 Silver Certificate blue seal and serial
1934 $10 Federal Reserve Note brown seal and serial			obverse: black "HAWAII" reverse: brown "HAWAII"
1934 $100 Gold Certificate gold seal and serial
1934 $1000 Federal Reserve Note green seal and serial			the basis for modern bills
1935 $10 Silver Certificate yellow seal, blue serial			obverse: blue "10"
1963 $2 Legal Tender Note red seal and serial

The following images, with overprints in subdued colors, are from the Bureau of Engraving and Printing.

Recent Federal Reserve Notes
2006 $5
2004 $10
2004 $20
2004 $50