Thanks, John. I did find the regimental history on both Gallica and Google books once I realized it was listed as "Historique des volontaires de l'Oise enrolés pour la défense de la patrie en ..." "Bibliographie des historiques des régiments francais, Volume 2" by Jean Hanoteau and Emile Bonnot proved very helpful. Page 156 is quite convincing. The 13th (all battalions) appears to never been anywhere near Hamburg in those year

Which is a bit of a surprise. The Suhr plate 80,
https://www.napoleon-series.org/military/organization/France/UniformsPlates/Suhr/Hamburg/c_SuhrHamburg1806.html
is captioned as the 13th, 14th, and 15th Ligne and used as evidence that the 13th wore white uniforms despite not being scheduled to change. The origin of the caption is not clear. (Markus?) The figure identified as the 14th is in error, as he should have black cuffs . Or rather he should not have both collar and cuffs in white. But if there is one error, why not two? Perhaps this is the 14th, 15th, and 16th (or maybe just one or two regiments). Certainly it appears the Suhr were not eyewitnesses to the dress of the 13th, and I'm surprised neither Rousselot or anyone else caught this.
Jon.