From "RCN Aircraft Finish and Markings 1944-1968" (paraphrased)
The RAF/FAA roundel in the 30's became known as the Type A roundel with bright red, white and blue rings, proportioned 1:3:5 and worn on top and bottom of both wings and fuselage. In 1941 the under wing markings were removed.
Dull colours came into play in 1939 as camouflage needed the markings to be harder to see, but the reduced visibility made it harder to identify freindly aircraft.
The Type A1 roundel corrected this by adding a thick yellow ring to the outside, proportioned 1:3:5:7. This happened about May 1940, but conflicted with the intent of camouflage, making aircraft too easy to see.
The Type B roundel (or night roundel, originially known as National Marking I) used the dull red and blue colours in proportions of 2:5.
This version proved to be too effective on the fuselage, so another changed was needed.
The white was reintroduced in a reduced size. The Type C roundel (National marking II) was red, white and blue in proportions 3:4:8 and used only on the fuselage sides and under the wings, while the Type B was used on top. However, it still wasn't visible enough so a thin yellow outer ring was added to aid in the recognition of friendly aircraft.
It is known as the Type C1 (National marking III) with proportions of 3:4:8:9 and was used on the fuselage only starting around mid 1942.
The main sizes of roundels were 18, 36 and 54 inch.
Thus the 'standard' remained until post war when a return to prewar markings was desired, but with a difference. Instead of going back to the Type A, the Type D was created with gloss Red, White and blue in proportions of 1:2:3 of the radius, giving a larger red center.
In Canada the Red center was replaced with the Red Maple leaf during the post war years. The attached photo is of a RCAF Crash boat in the late 40's or early 50's. The yellow ring is used on the dark hull, to aid in making the roundel visible and I suspect would have been used at the discretion of the CO.
Cheers!!
Darren