Hello High Wood
You might like to check out the MHS website,
http://www.themilitaryhistoricalsociety.co.uk, as one of their Special Publications is on "Coloured Field Service Caps, Infantry", cost £2.00 + postage. It is still available I believe.
I don't have a copy myself, so I can't say how good it is, perhaps another member can comment on this aspect.
If I remember correctly, there was also a wartime publication that dealt with Field Service Caps and cap badges with coloured line drawings. I had a copy once but without the title page, so no help as to the actual title, sorry!
Regards
Roger