I found his service record (what remains of it) on FindMyPast.
I used their free access this last weekend
It was part of the "Burnt Documents" series; TNA Classification WO 363.
Incidentally, the reason your searches are failing (IMHO) is that you are using incorrect terms?
As I said, the term "colours" refers not to uniform, but to the Regimental Standard (or Flag).
This term has been used in the Army since 1743; maybe earlier.
The Reserve Cavalry Regiments were formed at the outbreak of WW1, and never had a colour uniform. They only wore khaki.
