http://www.unofficialroyalty.com/royal-forums/13-spanish-royal-family/3223-the-queens-privilege-du-blanc?limit=6&start=6

'widow's weeds'

The CIA LAB became interested in who was allowed to wear WHITE and who had to be a 'widow' whilst visiting the POPE with her husband...and although there appear to be exceptions...on the whole - if you weren't Catholic and QUEEN - you had to wear the BLACK MANTILLA...

http://en.wikipedia.org/wiki/Privilège_du_blanc

...it is all rather strange...having to 'pretend to be a widow' because you aren't Catholic and a Queen...