As with most of the riddles that mention names, you need to find this one in the cemetery.
After you've solved the window in the crypt, go look at Beauregard's tombstone; it's in the first row, the first one on the left.
See those blank squares on the bottom? After you've solved the window, click on them, and Nancy will wipe the dust off to reveal --
-- some blank squares with colors. Huh.
Make a note of the colors, though; they'll be important when you get into Charlotte's bedroom.
How do I open the jewelry box?