The Spanish Academical Birrete
"... a biretta also of black velvet, with a silk tassel on the top in the colour representing the faculty to which each member belongs."
The number of mixed colors depends on the number of graduations the bearer holds.
Some academic colors used in Spain and Portugal:
White: Theology, Architecture
Green: Canon Law, Veterinary
Red: Civil Law
Blue Sky: Philosophy, Humanities
Black: official color of Spanish rectors
Blue (dark): Natural Sciences, Mathematic