After tapping on the wall, it opened automatically. Inside was another corridor - as damp as the previous one. "Eww!" exclaimed Cyrus.
At the end of the corridor, a huge stone tablet blocked the road. On the tablet were three symbols: a green horse, a blue mouse and a red river. Buttons were installed above the three symbols.
"Hmm... interesting symbols," said Cyrus.
Here comes the question. Which button should Cyrus press?
Hint: (must read)