The Collaborative International Dictionary of English v.0.48:

Isabella \Is`a*bel"la\, n., Isabella color \Is`a*bel"la col"or\
   [Said to be named from the Spanish princess Isabella,
   daughter of king Philip II., in allusion to the color assumed
   by her shift, which she wore without change from 1601 to
   1604, in consequence of a vow made by her.]
   A brownish yellow color.
   [1913 Webster]