Rousselot in plate #3 – line infantry 1804-1812, notes that the 1810 model shako had a neck cover "couvre-nuque" in leather, usually tucked-up inside the shako until needed. Later, he notes that on campaign the regiments often made use of shako covers, "confectionnes en toile ciree ou huilee." (Can someone translate?) He doesn't indicate a date, style or color in this plate.
Black oiled cover: In his plate #48, line infantry 1813-14, he shows five figures with covered shakos. All appear to be black oiled cloth. Two have chin-scales attached to the shako UNDER the shako cover (the disk does NOT show). Two do not show chin scales at all. The last has the neck cloth lowered, but it appears to be on a shako cover with the chin scale disks attached through the cover, showing on the sides. All are black with what appears to be a slight sheen.
Light brown cover and linen covers: His plate #62, officers of line infantry, he shows an officer on campaign in 1813 with a "latte" or "coffee with cream" light brown cover. He shows another officer on campaign in his plate #76 – light infantry – with what I would describe a natural linen, brownish-off-white cover.
Beige covers: Illustations related the Spain suggest a myriad color usage. Tradition Magazine #60 describes a voltigeur of the 16th light infantry in Spain (about 1812) with a "beige oiled-silk shako cover," no chin scales show. He also wears beige linen trousers tied at the ankles. Grey cloth half gaiters (how does he know? They are covered by the "balloon" trousers) with grey buttons. Black shoes.
This figure appears again in Haythornthwaite's Uniforms of the Peninsular War 1807-14 for the 16th leger, this time with a white neck cover hanging down below the linen-colored shako cover. Balloon trousers the same color as the cover. He appears yet again in "Military Dress of the Peninsular War 1808-14" by Windrow and Embleton, essentially the same colors. You have a lot of lattitude, for the two illustrations vary in color.
Light grey-green cover: On the same page in Embleton is an illustration of the Hessen Darmstadt regiment that defended Badajoz in the last siege. He is a grenadier with a shako covered in a light grey/green cloth. I would take German Feldgrau from WWII and add light grey to do this!
Tradition magazine #42 has an article about these Hessians. Grenadier with "grey-green cover with similar cover the the pompom (!!!!). Voltigeur with buff cover and green pompom. Fusilier – brown cover and double pompom – red top, white over yellow bottom pompom.
I have never seen a "brown" on an illustrated shako cover that is as dark as the Spanish sheep wool, but the French office mentioned above with the cream-in-coffee is approaching brown.
White covers: The Haythornthwaite book mentioned above shows a grenadier from Berg on campaign with a white shako cover. A Baden gunner on the same page has an earthy-linen colored cover. A soldier from Schwarzburg-Sonderhousen has a dirty white cover. A carabinier from Naples has a white cover with a black Roman numeral "I" on the front.
Light grey covers: I can't find the book, but I recall Osprey showing a soldier of the French 88th Line (a battalion was with the Hessians at Badajoz) with a light grey cover, with "88" in black with black laurels painted on the front. He wore white vest, and dark brown "balloon" trousers.
GdeP