I think (but not absoluetly sure) that the last campaign red tunics were used was in the Boer War. Someone suddenly realised that the soldiers stood out like a sore thumb, whilst the Boers (in a sort of green colour) were almost impossible to spot.
By busbys I think you mean bearskins as worn by the Brigade of Guards. Just a point.
My Grandfather fought in the Boer war (he relieved Mafekin with the help of others of course) and he wore khaki. Which, incidently is Urdu for dust.
Depends whart you mean by 'battle,' but there are references to red coats being worn on campaign in Zululand in 1888 and in the 4th Ashanti campaign of 1896...