Isabelline, sometimes called Isabella, is a colour, variously described as pale grey-yellow, pale fawn, pale cream-brown or parchment. In horse colours, Isabelline or Isabella Palomino are terms applied to very pale palomino horses whose coat colour is cream, pale yellow or almost white.
According to popular legend, the name comes from Isabella, Archduchess of Austria (1566–1633), daughter of Philip II of Spain (1527–1598). Her husband laid siege to Ostend in July 1601 and Isabella, expecting a quick victory, vowed not to change her underwear until the city was taken. The siege lasted a little over three years (ending in September 1604) and her underwear understandably became discoloured in the interval.
A more plausible, though probably still false, version refers to the much earlier Isabella I of Castile, the Catholic (1451–1504) and the eight-month siege of Granada by Ferdinand II of Aragon (1452–1516). This siege ended in January 1492 and again resulted in overworn underwear belonging to Isabella.