Smart thinking. This is a great way to respond to kids Gemma's age, who are bound to point out anything that's new or unusual to them -- including people who are missing limbs, have burn marks, or are shorter or thinner or fatter than other folks in their lives. They don't understand that what they say might be hurtful, and they don't do this out of malice -- which most people realize.
Even so, it can be mortifying for you, so when it happens:
Don't draw attention to the inappropriateness of the comment. That'll only humiliate more people, including the object of your child's fascination, who hardly needs more eyes directed at him.
Don't make your child apologize -- what he said may be true.
Do say something positive about the differences between people. You can boost your point -- and get your child's mind focused on something else -- by then asking, "And what color hair does your friend Julie have? What about your teacher?"
Do bring up the subject later. When there's a quiet moment -- in the car, say, or at bedtime -- explain that staring, shouting, or even just asking about the way a person looks can make her feel bad. Be sure to let your child know that you'll be happy to answer any questions he might have -- once the two of you are out of earshot.