Michael Barthorp's JSAHR article from 1978 "INFANTRY UNDRESS UNIFORM 1822-1902 deals only with regulation uniforms and local variations- i.e. in the case of forage caps- badges, peaks chin straps, etc.
Unfortunately, he specifically omits detailed discussion of khaki as "not strictly speaking undress."
The service kit issued to the Foot Guards for the Suakim expedition in Feb 1885 specified forage caps but these do not appear to have been a component of the khaki outfits supplied.
The Foot Guards being, of course, a law unto themselves as regards regulation uniform, never adopted the Glengarry worn by the rest of the infantry. The Guards Field Service cap (aka Albert bonnet) had been adopted just prior to the Crimean war and it seems this model remained in use by other ranks for fatigue purposes well into the second half of the C19th.
This cap, based on an Austrian model, with its turned up border, was a fore-runner of the 'Torin' style of cap seen worn increasingly by British officers in the field from circa 1880 onwards, first in Afghanistan, then in Sudan.
In Sudan, officers of the 3rd Grenadier Guards were photographed wearing 'Torin' style caps, (blue with coloured insets)
It seems most likely that, rather than a new style of cap being ordered, the other ranks of the Foot Guards contingent would have worn the existing model of blue cloth Field service cap, or a version of it, being wholly suitable for cold desert nights.
Khaki forage caps, mostly prototypes of the 'Austrian' model with the neck flap or 'hood' fastened with buttons made an appearance in the later 1880s,, as Frederick observed, but only worn by officers in India as far as I am aware.
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