A kid just asked me this question and the answer isn't as easy as you may initially think. Which is correct? The yolk of an egg are white or The yolk of an egg is white
As far as grammatically correct, "yolk" is an uncountable plural. "is" is correct. I do see A-Train's point.
I'd go with "The yolk of an egg is yellow". In fact, the word "yolk" is derived from the Old English word meaning yellow.