**Core Concept**
A green-colored intravenous (i.v.) cannula is commonly used for peripheral intravenous access in neonates and infants due to the smaller size of their blood vessels. The color coding of i.v. cannulas is standardized to prevent accidental use of a cannula that is too large, which can cause tissue damage or other complications.

**Why the Correct Answer is Right**
The green color of the i.v. cannula is associated with a 24-gauge needle, which is the smallest size available for peripheral intravenous access in infants and neonates. This size is chosen because it minimizes the risk of tissue damage and necrosis, while still allowing for adequate blood flow. The smaller gauge of the needle also makes it easier to insert and secure in the delicate skin of neonates and infants.

**Why Each Wrong Option is Incorrect**
**Option A:** A larger gauge needle would be more suitable for adults or older children, but not for neonates or infants due to the risk of tissue damage.
**Option B:** The blue color is typically associated with a 20-gauge needle, which is still too large for neonates and infants.
**Option C:** The yellow color is often used for 18-gauge needles, which are even larger and not suitable for peripheral intravenous access in neonates and infants.

**Clinical Pearl / High-Yield Fact**
Remember that the color coding of i.v. cannulas is standardized, but the gauge and size of the needle are specific to the patient's age and medical condition. Always verify the size and gauge of the i.v. cannula before use, especially in neonates and infants.

**Correct Answer: D. 24-gauge**