I had to look it up, but your blue potato guess made sense also. Delft blue, right?
Yep. Delft Blue was what I was thinking. WRONG ANSWER! ;)
The Jeopardy! Question is, 'What is the Carrot?'
The Dutch bred carrots to be orange in honor of King William the III, also known as 'William of Orange.'
Carrots were an unappetizing shade of pale yellow to begin with and now come in many colors, with bright orange being the standard.

Ha-Ha! Made ya LEARN something, today!