At this time (of William of Orange), the Dutch were primarily known as carrot farmers. And they grew carrots in the traditional hues of purple, yellow, and white. In the 17th century, a strain of carrot was developed that contained higher amounts of beta carotene -- the first orange carrot. Dutch carrot farmers started growing the new orange carrots in honor of William of Orange, and the traditional, more colorful carrots, were tossed aside for these newly fashionable orange carrots.
https://www.treehugger.com/why-carrots-are-orange-and-non-orange-carrots-grow-your-garden-4858707