I assume you are talking about British made Ford V8 engines.
Ford engines were painted Grey pre-war, but went to Khaki Brown when Military vehicles went into full pruduction. I don't know of a date for this, but my guess is early 1941. The Grey colour was highlighted in one of Bart Vandeveen's 'Wheels & Tracks' magazines.
In the Ford film 'Changeover' it shows vehicles being made, and these all have the engine the same colour as the vehicle. The film was made in late 1941 as it shows WOA2 vehicles but no WOT6 trucks are seen.
Ford V8 engine 7037774 is possibly very late 1942, but data on this is sketchy at best, so don't rely on this.
Not sure where the Bright Green colur comes from, but engines overhauled by Nordians in the 1950s and 1960s were painted a bright Greeny-Bluey colour, and Fordson Tractors were painted green.