There's a couple of misconceptions in your question. Firstly, warships in WWI and the interwar period were, largely, painted in simple schemes. As I outlined in an earlier answer here, camouflage on larger warships were mainly short-lived experimental schemes. Dazzle camouflage was applied to patrol vessels and escorts, but these made up a relative minority of the fleet. Most of the fleet would remain painted in a single shade of grey. In the interwar period, meanwhile, warships were usually painted in single colours. In the RN, Home Fleet ships were painted in a dark grey/blue colour, designated 507B. Ships on the Mediterranean or North American stations were painted in the lighter grey of 507C. Ships on tropical stations were painted with white hulls, with their upperworks painted in a variety of light shades - gray on the China Station, or white with yellow funnels on the East Indies Station. The outbreak of WWII resulted in more camouflage schemes being adopted. As described in another earlier answer of mine, these started off as ad-hoc or experimental schemes, but slowly developed into centralised schemes produced by the Admiralty's Camouflage Section.

Given this history, shifting back to grey schemes in peacetime makes sense. It was a return to the standard peace-time schemes. However, this does not tell the whole story. Towards the end of WWII, we can see a simplification of the camouflage schemes. Compare this photo of Anson from 1942 with this one of her from 1945. In the former, the camouflage scheme is complex, with large blobs of multiple colours. In the latter, though, the camouflage scheme is much simpler, just two colours; the ship has a dark bar down her hull just above the waterline. The reason for this simplification comes from the technical advances over the course of the war. Aircraft spotting became a lot more important as the war progressed. From the air, a ship's wake could easily be spotted, whether or not the ship was camouflaged. Radar (and, to a lesser extent, sonar), meanwhile, made camouflage totally irrelevant; a camouflaged ship would stand out on a radar screen just as much as a non-camouflaged one. As such, simpler schemes were adopted. These were generally found to be more useful against aerial spotting, as well as against sea-level observers at dusk, dawn and at night. An all-over grey scheme, though, still provided useful camouflage, and was an approved emergency scheme if the paint for the standard schemes was not available. In the post-war period, the standard grey colour was chosen such that it maximised the effectiveness of the camouflage, while remaining easy to paint and maintain. Camouflage was, however, not seen as a key part of naval warfare, as there was a much greater reliance on detection by electronic means. During the Falklands War, the only British ships that were repainted were painted to make them more visible - both the Argentinian and British navies operated the Type 42 destroyer, so the British Type 42s were given a large vertical black stripe from their hulls to their funnel to ease identification.