The painting of ships (of more or less any navy) during WW2 is a very complicated subject and I don't have enough knowledge of the subject, but I can certainly tell you that both these ships carried numerous different paint schemes during the war, including both overall grey and pattern camouflage
The paint scheme on the preserved HMS Belfast (and in the Airfix kit - which I assume is what you're building, it's the only plastic Belfast kit around!) is accurate for its appearance at the Battle of the North Cape (sinking of the Scharnhorst) in 1943. However, at the beginning and, I believe, the end of the war, it did have an overall grey paint scheme. I'm sure it carried numerous other schemes during the war.
Unfortunately, unlike US Navy ships (where there is an excellent website http://www.shipcamouflage.com as well as several good reference books) there is not a great deal of readily accessible detailed information on British Royal Navy WW2 camouflage and paint schemes. However, looking at the archive photos on sites like http://www.battleships-cruisers.co.uk and model galleries such as http://www.modelshipgallery.com/gallery/index-gallery.html should give you some inspiration!