An interview was conducted to test the structure, usage and competence of all the English Letters. To do this, each letter was interviewed for one hour in sequence and the interview was expected to take a whole day. Letter A was the first, then B, C, D ... till letter X which rounded off the interview. Why was letter Y and Z left out?

What is
black when you buy it,
red when you use it,
and gray when you throw it away ?