My parents referred to egg yolk as "YATTI". They say my grandmother from San Antonio called it Yatti. Not sure of the spelling, but does anyone have any idea of the origin of such a phrase for the yellow in egg yolk?

Well, it isn't Spanish.  What is your family's ethnicity?  Sometimes, families have words because a child mispronounces a word and every just picks that up.  Your grandmother may have just called it yatti when she was a little girl and it became a family in joke.  That's what makes families unique.