a flag is a communication tool. a basic and common function is signalling if a path is clear (green) or obstructed/hazardous (red).
imo, many people are using "flags" weirdly because it makes more sense to apply it to behaviors and attitudes rather than to a person or character as whole. that's how I saw the concept used, initially, anyways. so this is in terms of attitudes, behaviors and actions
green flags: emotional intelligence (being able to know/identify one's own feelings and recognize them in others), respect for boundaries, willingness to share, communication skills, anything that facilitates a functional relationship
red flags: being controlling, ignoring and violating boundaries, undermining the autonomy of others, superiority complexes, being invested in hierarchies, self-sacrificing or guilt-tripping, anything that leads to an abusive relationship
stuff like that
you can have conflict in a story, even a romantic one, without resorting to abuse and so-called red/black flag characters. it's as simple as characters having different needs and wants in life, or pressure from external forces. but that kind of thing takes actual writing skills and capacity for complex reasoning.
(there's a question about flags that I'm responding to, but it looks like I'm blocked by the op of that)
