While this is certainly true for the British 1820-1850s after Crimea and during the mutiny functionality, practicality begin to take more prominence for service uniforms. Not that the “old red rag” wasn’t worn but post Crimea you certainly see Napoleonic pomp losing out to things like terrain, weather and protection. Even as early as the 1840 Kaffir wars the British were saving their parade jackets and fighting in cut down plain shell jackets.
AFAICR the rank and file ALWAYS wore a dark, brick red ... the brighter scarlet was used for officers and sergeants, certainly during the 1790-1820s.
Post Crimean War saw a major rise in governmental paranoia about a possible French invasion, largely due to having seen the efficiency French organisation and arms close up. This occasioned a major building and reinforcement programme for coastal defences, a review of the army and the creation of the Rifle Volunteers who were not regular troops but neither were they yeomanry or militia.
Volunteer Rifle brigades were raised all over the country (they aimed to recruit primarily from the numerous shooting clubs) as a kind of Home Guard defence force to be mobilised in the event of invasion. They were assigned experienced NCOs to drill with but their officers were hastily gazetted civilians drawn from the local middle and upper classes. On formation uniforms were usually a plain grey (or occasionally blue) with coloured facings and as much or little frogging and knotwork as felt necessary by the local CO. Individuals were initially expected to supply their own rifle which had only to be the correct calibre although most ended up carrying the 1853 pattern Enfield rifle musket.
As a consequence they look extremely similar to troops from the American Civil War and almost entirely unlike what we might expect a British unit of the time to resemble.