Traditionally, green Thai curries are spicier than red curries. Authentic Thai sources support this statement, explaining that Kaeng khiao wan is often far spicier thanks to the addition of bird’s eye chillies rather than long chillies.  The belief that red Thai curries are hotter than green ones might have stemmed from the common human perception that green is a cooler color than red, and that green foods are logically less spicy.  Some misleading articles online, written by writers unfamiliar with Thai cuisine, also help in perpetuating this misconception.