The seal colors mean the same thing whether the money is uncirculated or has been spent.
Seal colors weren't standardized until the 1920s so the this answer only applies to bills printed since then.
- Red seals designated United States Notes, a form of paper money issued directly by the federal government rather than through the Federal Reserve System. US Notes were issued from 1862 until 1971, although the last ones were dated 1966. They functioned identically to Federal Reserve Notes and were eliminated in order to reduce costs by standardizing designs.
- Blue seals are characteristic of silver certificates. Silver certificates were backed dollar-for-dollar by silver metal held by the Treasury, so their printing was limited by the amount of metal available to back them. Silver certificates were discontinued when the price of silver was deregulated and the government was forced to sell its stockpile.
- Green seals indicated familiar Federal Reserve Notes. These are bills printed and distributed under the authority of the Federal Reserve System, which today is responsible for all US paper currency.
There's never been a silver seal on standard US paper money.
Other colors:
- Gold: Gold certificates were similar to silver certificates except they were backed dollar-for-dollar with gold. They were discontinued when the US went off the gold standard in 1933.
- Brown: Brown seals have been used at various times for a short-lived type of money called National Currency, and as a security measure for special bills printed during WWII for use in Hawaii.
- Yellow: Yellow seals were used on special bills printed for use by troops in North Africa during WWII.
