Which colour needle should I use when vaccinating patients?

When considering which needle, the main thing you need to worry about is the the patient. Which needle would cause the patient the least distress? Think about it, the most common side effect to any vaccine is erythema which is caused by localised damage to tissue. You should then ask yourself how this can be minimised? 

If you use an orange needle, this will restrict the bore size of the liquid vaccine coming out of the needle causing more dispersal pressure at the vaccine site i.e. into the muscle or subcutaneous fat. To reduce this, a larger bore should be used, such as a blue needle.

An easier way to understand this, is the classic garden hose analogy. Imagine you are in your garden watering your plants with a garden hose. You want the water to reach a bit further in the garden so you restrict the width of the hose with your thumb, increasing the pressure so the water can reach a further distance. Your finger on the hose, reduces the aperture, the bore and essentially increases the pressure at the site of origin. Imagine then, if you use this reduced bore size on the soil closer to you, it will disperse the soil more than if the bore was wider. This is essentially the same as with muscle. If you use a smaller bore size this will cause more localised erythema.