Trimeresurus similarities

Can someone explain to me what (if anything) definitively separates Trimeresurus nebularis, T. hageni, and T. sabahi fucata in peninsular Malaysia? T. sabahi USUALLY has a red ventrolateral stripe, but not always (usually depending on sex). It also sometimes exhibits black interstitial skin, as does hageni and nebularis. Additionally, they all display blue-ish labials and all occur sympatrically, if not syntopically.