E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
The colour so called is the yellow of soiled calico. A yellow-dun horse
is called in France un cheval isabelle. The tale is attached to Isabel of Austria and Isabel of Castile. It is said that Isabel of Austria, daughter of Philip II., at the siege of Ostend vowed not to change her linen till the place was taken. As the siege lasted three years, we may well suppose that it was somewhat soiled by three years wear.
His colour was isabel, a name given in allusion to the whimsical vow of Isabella Clara Eugenia, Governess of the Netherlands, at the memorable siege of Ostend, which lasted from 1601 till 1604.Dillon: Travels in Spain (1781).
Isabel of Castile, we are told, made a vow to the Virgin not to change her linen till Granada fell into her hands; but this siege lasted longer than ladies are wont to wear their body-linen.
Bright-Sun was mounted on a black horse, that of Felix was a grey, Cherys was white as milk, and the princesss an isabelle.Countess dAlnois: Fair-star and Prince Chery.