A color which is yellowish or buff. The name comes from the Princess Isabella of Spain, who made a vow that until the city of Ostend was conquered by her husband Archduke Albertus, she would never change her clothes. The siege took three years, so Isabella's clothing was a new color by the time it was over. (My source, the Forgotten English page-a-day calendar quotes from an 1819 book, which does not give the date this went on.)