They are military colour codes used as shorthand indications of basic met conditions at the field
The information given by a colour code is:-
Surface vis: RN km (RAF nm) [USAF km/statute mile] - base of lowest cloud layer of 3/8 (sct) or more.
Blue 8.0 (4.3) [8.0/5.0] - 2,500 ft agl . .White 5.0 (2.7) [5.0/3.0] - 1,500 ft agl . .Green 3.7 (2.0) [ 3.7/2.3] - 700 ft agl . .Yellow 1.6 (0.9) [1.6/1.0] - 300 ft agl . .Amber 0.8 (0.4) [ 0.8/0.5] - 200 ft agl . .Red Less than Amber vis - below 200 ft or sky obscured . .Black (+ colour) = field not usable for other reasons.