Flag adopted c. 1806, abolished 1866
Andorra's current flag was introduced in 1866 by adding a blue field to the yellow and red flag of Foix. By doing so, both France and Spain were represented with two colors in Andorra's flag (red and blue for France and red and yellow for Spain).
Source: Herzog and Hannes 1990
Volker Mörbitz Keith, 11 Apr 2000
The source of my image is the flag plate in Grand Larousse Illustré du XXe Siècle, 1932. Since several sources stated that the Andorran tricolor flag was adopted at the end of the 19th century, one may wonder why Larousse showed such an outdated flag, especially for a country which is geographically and politically so close to France. Talocci 1993 says the current tricolour flag replaced the former yellow-red flag in 1866, and attributes the new design to Emperor Napoleon III (reigned 1852-1870). The only source I know mentioning the adoption date as 1806 is Pedersen 1971, who says the colours were those of the Counts of Foix, in agreement with Herzog and Hannes 1990. Finally, there is a question which remains to be answered, what was the flag of Andorra before 1806?