Apart from the colour, the difference is in the heat. Thai red curry is made from red chillies (which are hot) and Thai green curry is made from green chillies (which are milder). The colour and flavour of Thai green curry is further enhanced by the adddition of greenery in the form of basil, coriander and kaffir lime leaves. There is a third variant, Thai yellow curry, which sits somewhere between red and green Thai chilli on the heat scale, and is made from yellow chillies enhanced with turmeric.