I don't see in this way...in S1 sesha was positive in first half though she was a black snake she changed after falling in love with ritik and in S2 she was a black snake than turn into red thakshak snake than purple snake(2 or 4 mukhi snake)
in S3 vish was a red snake and in the end she turns into 2 mukhi snake and turns out to be she too was after the naagmani
i never go by the colour at all,it changes as per track,sesha went through a colour spree
I actually see through a naagin attracts her soulmate it can be a human or naag and i determine that yes the naagin is a positive character...shivanya was a naagin and ritik was her soulmate thus he got attracted to her at first sight and that goes same for rocky too he was shivangi's soulmate and he got attracted towards at first sight and same goes for mahir in S3 and in S4 though Brindev knows each other from childhood but dev is besotted by brinda
if go by the track ritik rocky mahir and now dev is never attracted or besotted by sesha vish and nayantara...a naagin definitely attracts her real soulmate and thats how i determine the naagins or positivity and negativity,this is my way actually
So in other words the naagin that gets the guy is positive by default and the one who doesn’t is supposed to be a vamp that obsessed over both the guy and the naagmani?