A good question.
The use of khaki - as you quite rightly say officially drab - was an attempt to encompass the variety of uniforms on which the cloth insignia shown in the book was worn.
The book starts with the adoption of drab Service dress in 1902 then goes on to explain its use in WW1 and the interwar years as many of the badges adopted for battle dress had earlier origins. The wartime use of such the badges then spread to the forces overseas who wore khaki drill and later jungle green, all eventually with most of the same badges worn on battle dress.
It also looks at the use of battle dress by the Royal Navy, Royal Marines and RAF as all later adopted khaki battle dress and versions in their own colours and by the wide variety of civilian organisations which served alongside the armed forces from War Correspondents to Welfare Workers.
To contact me outside the Forum if you have questions email
bob19391967@yahoo.co.uk
Jon