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

Concept: In a renaming (object-substitution) coding puzzle, each rule "X means Y" tells you that whatever object is truly the colour X in the real world is written as Y under the code. To decode any object's colour, first find its real-world colour, then follow the single rule whose left side is that real colour to read off its coded name.

Applying it here:

1. Blood's real-world colour is red.

2. Find the rule whose left side is "red": the rule states that red means yellow.

3. So the true colour red is written as "yellow" in this code.

Cross-check: The same one-hop lookup works for any object -- for example, the sky's real colour is blue, and the rule "blue means orange" gives its coded name as orange, following the identical real-colour -> matching-rule -> coded-name pattern used above for blood.

Result: the colour of blood is coded as yellow.