Personally, I would love to see a world where we didn't feel the need to comment on someone's appearance so much. Even complimenting someone can make them feel like their value is in their looks. So, describing someone's appearance, just as a description, is fine, but we should stay away from adding value to that, e.g. they have long, dark hair, v. lovely dark hair.
Depends what age your dc are, who it is, context etc. a casual comment in the park is hard, but if you're discussing it with someone you know you can get into talking it through more.
If your dc are young enough to do this, you could do the 'repeat with a slight change' thing, e.g "Yes, she/he does have long dark hair" rather than "lovely dark hair" etc. I would think that the aim is to let dc grow up knowing that their features are recognized and accepted, without being made to feel that their self worth is associated with them. So, describing them, and discussing the background to how they inherited their features is part of who they are, their history, but saying that it's nice/not nice can cause insecurity, because they have so little control over things like eye color, hair, skin, height etc. Ascribing value to something a person can't control is an 'ism' (racism, heightism) etc. and something that happens ALL the time (aren't you nice and tall etc). There will be nothing you can do about hearing it, but how you react, and talk it through with dcs will be important.