**Core Concept**
Nitrous oxide is a colorless gas used as an anesthetic and analgesic agent. The color of the cylinder is not related to the gas itself but rather to the color-coding system used for gas cylinders to ensure safe handling and identification.

**Why the Correct Answer is Right**
The color of the nitrous oxide cylinder is blue. This color-coding is part of the British Standard for the color-coding of medical gas cylinders, which is widely adopted in many countries. The blue color helps to prevent errors in gas cylinder selection and ensures that the correct gas is used for the intended medical procedure.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not match the actual color of the nitrous oxide cylinder. The color of the cylinder is a critical safety feature that must be accurately identified to prevent errors.
**Option B:** This option is incorrect because it is not the color used for nitrous oxide cylinders. Other gases may have different color-coding to distinguish them from nitrous oxide.
**Option C:** This option is incorrect because it is not the correct color for nitrous oxide cylinders. The color-coding system is specific to each gas, and using the wrong color could lead to serious errors.

**Clinical Pearl / High-Yield Fact**
The color-coding system for medical gas cylinders is a critical safety feature that must be accurately identified to prevent errors. Always verify the color of the cylinder before selecting the gas to ensure safe handling and use.

**Correct Answer: C. Blue**