Hi Freddie
To my understanding the temperate sea scheme (dark sea grey/slate grey) with matt white fuselage and gloss white undersides came into use in July '42 so the chances are this would be applicable unless the unit was serving in the Mediterranean -then dark earth/midstone with sky undersides would apply. Some aircraft kept the fighter colours (dark earth / dark green)
See: https://www.hrmtech.com/SIG/coastal_cam.asp
As per Wikipedia 203 Sqn was allocated to North Africa and the Med, so the dark earth / midstone / azure blue blue scheme may apply (same used by SAAF aircraft):
"Shortly before the start of the war the squadron was re-equipped with Short Singapore III, long-range maritime patrol flying boat.[7] and in 1940 with Bristol Blenheim, a twin-engined monoplane light bomber. The squadron flew patrols over the Red Sea from Basra. At the end of 1941 the squadron operated Bristol Blenheim IV, undertaking reconnaissance over the Mediterranean from various bases in Western Egypt, flying patrols from the Libyan coast out as far as Crete. In 1942 the squadron re-equipped with Martin Baltimore, an American twin-engined light attack bomber, also used as a reconnaissance aircraft and was involved in operations in Syria." They were based from RAF Ismailia (landng ground 206) which was in the desert.
Also see: https://raf203squadron.wordpress.com/
Hope this helps you
Regards
Tim H