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What color was the rightmost ring in the Olympic flag again?
Literal
Olympic flag [attributive-の] right-end [attributive-の] ring [quotative-って] what-color was [reflective-っけか]?
Casual memory-dredging question. 五輪 ('five rings') is the standard Japanese term for the Olympics (the five-ring symbol). 輪っか is a casual/childish word for 'ring, loop, hoop' — cognate with 輪. ~っけ is the memory-prompt particle ('was it X, as I recall?' — used when the speaker is trying to remember). The stacked ~っけか adds questioning uncertainty. って is the casual topic marker.