Here is the 'Isabelline Shrike' a smallish bird that lives mainly in central Asia but can be found across Europe and North Africa as well. 'Isabellinus' is its species name; the word refers to its color.
And here is Ursos Arctos Isabellinus, the Himalayan Brown Bear. There are bugs and bats that are isabelline as well as another few birds. So, what kind of brown is Isabelline?
The Isabel in question is Isabella I, Queen of Castille, the queen that sent Columbus off as her privateer to gain access to the Indies. For centuries previous, the christian rulers of various kingdoms on the Iberian peninsula had been scheming against each other but occasionally they would take a moment to think about kicking their Moorish neighbors to the south out. The muslim dynasty that controlled southern Spain was an aging superpower by the 1400's, and when Isabella and her second cousin Ferdinand got a dispensation from the Pope so they could marry they united two of the more powerful christian kingdoms, Castille and Navarre. For a decade the wars raged until the last holdout was the city of Granada, and legend tells us that at this point Isabella swore she would not change her undergarments until her christian forces were victorious. So that's what kind of brown Isabelline is.
From the year 1600, here is an item from Queen Elizabeth's wardrobe itinerary: "one rounde gowne of Isabella-colour satten, cut in snippes and raised up, set with silver bangles”.