Grey paint was the general rule for Allied merchant vessels. Some ships had funnel tops and top half of masts painted white so to blend with the sky.
Apparently there was no official rule prescribing the above, so the grey paint was more a matter of common selse by shipmasters/shipowners than else.
As a consequence of the previous point, there was not a sudden transition from peacetime colours to wartime grey. Some ships – probably the ones whose trading routes were closer to the main war theatres – were repainted at the earliest opportunity after the war broke out, whereas a few others are reported to have switched colours as late as January 1941.
Ships built during the war were delivered in grey paint. Late in the war a few Liberty ships might have sported dazzle comoufflages too, but that was not universal.
The implementation of safety rules was somehow more strict for ships sailing in convoys. These rules included:
avoiding bright hull/superstructure colours;
not having the ship name painted on the hull;
no dark funnel smokes (this would have ruled out old coal-burning steamers from convoys).