Probably not. Those colorful European uniforms are from the movies, like Gettysburg, and The Blue and the Gray. Col. Fremantle wore civilian clothes.
Most Civil War reenactors laugh at people who show up as Prussians or British Colonels in full uniform or even "undress" kit and swan about at events. One event in Virginia drew a Prussian Lancer officer and a "Vivandiere" from his unit along with a Mexican observer in his sombrero. The history just doesn't show observers commonly wearing this kind of dress in the field.