The picture below comes from Napoleon Series and is by the Spanish artist Dionisio Álvarez Cueto.
The best site that is available (IMHO!) is Steve's Balagan from which I lifted the following text and table.
I also have a link in the sidebar under Spanish War of Independence (1808-1814) which leads you to his website as well.
1806 Light Infantry
(Funcken, 1973; Haythornthwaite, 1995; Sapherson, 1991). The key differences were:
|Type of Regiment||Coat||Turnbacks||Waistcoat||Breeches *||Piping||Gaiters||Plume|
|Light||Dk. Blue **||Regimental||Dk. Blue||White||Regimental||Black||Green|
* Breeches were often replaced by long brown or checked trousers.
The Light Infantry regiments also had regimental colours for Turnbacks and Piping (Funcken, 1973; Haythornthwaite, 1995; Sapherson, 1991). The light infantry often wore their brown greatcoat in the field. Similarly they often only buttoned up the front of the bicorn and left the back to drop down over their neck. Note: Sapherson uses the term "Carmine" where Haythornthwaite and Fucken use "Crimson" instead.
|#||Light Infantry Regiment||Collar||Turnbacks, Cuffs & Cuff flaps||Lapels||Buttons||Piping||Remarks|
|1||Aragon||Red||Red||Red||Silver||Red||Haythornthwaite (1995) says red piping but Funcken (1973) says white.|
|2||Aragon||Dk. Blue||Red||Red||Silver||Dk. Blue||Haythornthwaite (1995) says silver/white buttons, but Sapherson (1991) says gold/yellow.|
|Vol. de Navarra||Crimson||Crimson||Dk. Blue||Silver||Dk. Blue||Haythornthwaite (1995) says Dk. Blue collar, but Sapherson (1991) says Carmine/Crimson.|
|Vol. de Valencia||Crimson||Crimson||Crimson||Silver||Dk. Blue|
|Campo Mayor||Dk. Blue||Crimson||Crimson||Silver||Dk. Blue|
|Barbastro||Red||Red||Dk. Blue||Silver||Dk. Blue|
Now I have to decide which regiment I want to paint up!