Blue "pops" more visually than red which is a low visibility color, and the brightest blue would best assert itself as on the Triumph above. A "refrigerator white (there are many whites) would make the blue and the roundel stand out most.
RAF (might as well do it correctly) roundel choices. Those with yellow rings were obviously chosen for contrast and would look good on a blue or white background.
Parent page has the other roundels: https://en.wikipedia.org/wiki/Royal_...C1_Roundel.svg
Gratuitous pics of something else with yellow-trimmed roundels which shows their contrast well:
RAF (might as well do it correctly) roundel choices. Those with yellow rings were obviously chosen for contrast and would look good on a blue or white background.
Parent page has the other roundels: https://en.wikipedia.org/wiki/Royal_...C1_Roundel.svg
Gratuitous pics of something else with yellow-trimmed roundels which shows their contrast well:
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