If this is a US extension cord, never trust the colors. Get an ohmmeter or continuity tester, and see which wires are connected to which pin on the plug. If you do not have either of those, ask a friend or neighbor who does. The round pin on the plug is ground, and is probably the green wire, but don't trust - check it. The wider flat blade on the plug is neutral. And the narrower blade is hot (aka line).
If another country, please tell us which one.