National markings have been worn on military aircraft from the earliest days of the First World War when it was realised that the accidental shooting down of friendly aircraft was probably just as likely as deliberately destroying an enemy. After initially using a red on white St. George's cross insignia which at distance bore a similarity to the German black cross, this was quickly changed to a tri-coloured roundel with it's colours reversed to the French allies. A red centre, surrounded by a white circle, surrounded by a blue circle forms the basis of the roundel which is still in use today.
Usually worn on 6 locations (fuselage sides, plus above and below each wing), the roundel is also supplemented by a 3 colour vertical flash on the fin or rudder, with red being the leading colour.
Later variations to the basic marking were made to tone down the roundel in order to avoid it from spoiling camouflage schemes. The red and blue colours were made less vivid, and the white circle thinned down considerably when initially used on night fighters during the second world war. A thin yellow outer circle was later added to this roundel when it was painted on more tactical types, and even later both the yellow and white were removed, initially on high altitude photo-reconnaissance types. After the war the traditional 3 coloured roundel became more in vogue for a while, but with the cold war reintroduction of camouflage the 2 colour tactical roundel became the standard. The V-bomber force introduced an all white anti-flash colour scheme, and this meant that the three-coloured roundel had to be toned down to a pale blue, white, and pink version. These colours made a re-appearance on the all-grey air-defence types in the 1980s in a small 2 colour pink and pale blue roundel.
Today the 3 coloured roundels are worn on training and high visibility aircraft types, the 2 coloured red and blue tactical roundel is worn on generally darker camouflaged tactical aircraft types, whilst the pink and light blue roundel is worn on the light grey camouflaged air-defence types.