Q. While driving in Illinois last week I saw a number of purple trees. Can you tell me what these trees are?
Those trees were most likely Paulownia trees. The genus Paulownia was named in honor of Anna Paulowna, a princess of the Newtherlands, who died in 1865. The empress tree Paulownia tomentosa originated in China and was imported into this country for cultivation and has become naturalized in the eastern part of the United States. It's common to see Paulownias in bloom in April. See link below from Google showing a page of images of the tree and its blooms.
A blog post recently appeared on the Growing History blog. I am adding a link below to that post. You may enjoy learning some more about the history of the empress tree.