There are two boxes found in HKs - early ones had the grey and later ones the black. Others will be better placed than I to give you an estimate of the changeover but my observations suggest from approx October 68 on, black was used. The grey ones appear to be some type of plastic while the blacks look more like fibreglass

I don't believe either was actually painted - rather, it is from the colour of the plastic/fibreglass resin used in their manufacture.