I asked the met office the same question lst year and received a very helpful response:
METAR reports from military airfields operated by the RAF, some USAF and others may have a 'colour-code' appended (usually only when ATC is open), which describes the airfield 'fitness': these run from BLU best, through WHT GRN YLO (1 and 2), AMB and RED. The colour is based on the lowest cloud base (usually 3 oktas or more cover, but some use 5 oktas) and the horizontal 'MET' visibility. BLACK is also used, for airfield closed for non-weather reasons.
For reference:
Black - airfield closed for non-weather reasons
Red - cloud < 200ft and/or visibility < 800m
Amber - better than Red, but cloud < 300ft and/or visibility < 1600m
Yellow - better than Amber, but cloud < 700ft, and/or visibility < 3700m
Green - better than Yellow, but cloud < 1500ft, and/or visibility < 5km
White - better than Green, but Cloud < 2500ft, and/or visibility < 8km
Blue - cloud > 2500ft and visibility > 8km
Where 'cloud' is the lowest cloud base designated as SCATTERED or worse
I hope this helps and thank you once again for your interest.
If you have any further questions or need additional information please contact the Weather Desk on 0870 9000 100 where one of our advisors will be happy to help you. The number is open 24 hours a day, seven days a week.