To solve this question, the user needs to have knowledge about the colors of different gases.

Option A: Chlorine is a greenish-yellow gas at room temperature and pressure. Therefore, option A is a possible correct answer.

Option B: Hydrogen is a colorless gas at room temperature and pressure. Therefore, option B is incorrect.

Option C: Krypton is a colorless gas at room temperature and pressure. Therefore, option C is incorrect.

Option D: Neon is a colorless gas at room temperature and pressure. Therefore, option D is incorrect.

The Answer is: A