The re-enactors will probably pounce on me - but weren't the issue field mugs a dark-ish red-brown colour ( almost brick red ) ?
G'day Have seen several colours. The above mentioned brown mug in several shades of brown, but also white and green-ish. They are marked WD and year. Have a postwar 1953 one for re-enacting. These are much stronger then the available mug's sold for re-enacting. Popski
As far as I know they were usually brown though white ones were produced and distributed (usually not to combat troops though I am guessing) as well as sent to the Russians through lend lease.
G'day I think that a mug was a mug in those day's and not get too concerned about the colour. The pre-war mugs were white but later on they got some brown colour according to B Davis. Here a picture on road to Arnhem with white enamel mug hanging on the small packs. Just as long as you can brew up. "When in doubt, brew up " Cheers