If green means red, red means yellow, yellow means blue, blue means orange and…

Concept: In a renaming/substitution-based coding question, every real name is replaced by a code word according to a stated rule. To find the answer, first pin down the real-world fact being asked about, then trace that real name through the given substitution chain to find its assigned code word — the final answer must be given in code, not in the real name.

1. Write out the given substitution chain: green means red, red means yellow, yellow means blue, blue means orange, and orange means green.

2. In reality, the colour of a clear sky is blue — this is the real-world fact the question is really about.

3. Find which code word has been assigned to the real colour blue. Scanning the chain, "yellow means blue" — so the code word standing for blue is "yellow".

4. Therefore, expressed in the given code, the colour of the sky is "Yellow".

Cross-check: in the chain green → red, red → yellow, yellow → blue, blue → orange, orange → green, exactly one arrow has blue as its target — the arrow yellow → blue. So only one code word can correctly stand for the real colour blue, confirming yellow is the unique, correct code word and no other option can also be blue's code.

Hence, the colour of the sky, in the given code, is Yellow.