There is always carbon left from burning gas (expecially with radical camshaft where mixture atomization is poor at low engine speeds). If your engine/combustion chamber isnt hot enough it will stay there (carbon in plug). When you drive at higher load then plug temperature goes higher and it shows its "real" color and if your mixture is right and plug temp rate right then the color is coffee brown. When I checked plug I drove like 20 miles per hour for some time, it explains why plug was black.
