That Osprey plate depicts a soldier wearing a red night cap. There are examples of forage caps in the other plates.
Forage caps for the period were principally of the broad crowned variety and came in either blue or green (for Rifle Regiments), although the pork pie type became increasingly common. At any rate both patterns soldiered on for some time. When they were first issued, they had a band that matched the facing colour of the regiment concerned or red if it was a Royal regiment. By the 1840s they were mostly issued in plain blue or green, with no other distinction save for maybe a regiment cap badge. There were exceptions, some regiments still wore the older style caps with coloured band and Highland Regiments wore diced bands on them.
From the mid 1830s forage caps were authorised to be worn with peaks for soldiers in hot climates. They are frequently depicted with white cotton covers in India and the very occasional black oilskin cover (there's an illustration of an example at the back of that book). In South Africa they frequently had a rear peak as well, kind of mirroring the Albert shako and as depicted in the Empress Orange River range. Covers don't seen to have been worn much in South Africa.
As for nether wear, the exigencies of campaigning anywhere, miles from regimental stores meant that substitute items would appear and lots of patching took place in contrasting materials and colours. Again the Osprey has a good set of first hand accounts of the variety worn.