A Reuters story that I found on ESPN has all the details, including the fact that Señor Aragones "refused yellow flowers" when his team arrived in Dortmund for 2006 World Cup. (The team colors of Borussia Dortmund are the same black and yellow that you see on the strange smiley face ball depicted here.)
Aragones also threw a fit in 2006 when Spanish striker Raul dared to wear a yellow shirt to practice.
And let's not forget to blame the theatre! The story ends with this truly bizarre cross-cultural explanation of the superstition -- a French playwright's death:
The traditional Spanish superstition surrounding the colour is thought to have arisen because French playwright Moliere was said to have worn yellow when he collapsed on stage performing Le Malade Imaginaire in 1673. He later died at his home.
And yes, please ignore the fact that the jerseys are really sorta gold.
Yellow Smiley Face Super Ball by Flickr user L. Marie used under a Creative Commons License.