The dictionaries say is refers to a greyish-yellow colour, though it has also been used as the name for the colour of parchment or sand. It’s clearly one of those intermediate or indeterminate colours for which the creators of paint catalogues must search creatively to find a good name. They haven’t borrowed isabelline, however, which went out of use in the nineteenth century, except in the fixed names of a few animals and plants, such as the isabelline wheatear, the isabelline shrike, and the isabelline bear, which is a reddish- or yellowish-brown animal of the Himalayas.
The word clearly comes from the personal name Isabella. There’s a folk tale — mentioned in the Oxford English Dictionary only to deny its truth — that says the origin was Isabella, Archduchess of Austria, daughter of Philip II of Spain. He laid siege to Ostend in 1601 and in a moment of filial fervour Isabella vowed not to change her intimate undergarments until the city was taken. Unfortunately for her (and for those around her) the siege lasted another three years, leading to this off-colour word for over-worn underwear.
It’s easy to save the lady’s reputation, as the name’s recorded in an inventory of the wardrobe of Queen Elizabeth I a year before the siege began, in 1600: “one rounde gowne of Isabella-colour satten ... set with silver bangles”.
However, subscribers tell me that the word is also known by related names in French, German, Spanish and Italian. Its sense in French and German primarily refers to the colour of a horse. These languages have much the same folk tale about Isabella’s underwear. However, the references in all cases are to the siege of Granada by Ferdinand and Isabella that ended in January 1492. It seems that some tellers of the tale may have seized upon the wrong Isabella.
One explanation for the origin of the word is that it derives from Arabic izah for lion, so roughly “lion-coloured”.