Traditionally, all Thai curries were made with the same ingredients except for one thing: the chilies. Red curry was made with up to 20 red chilies for a fiery hot dish, while green curry was made with green chilies and yellow curry was made with yellow chilies. In Thailand, these chilies have slightly different taste characteristics in addition to their color.
Over time, other ingredients were added to enhance each of these curries, making them more distinct.
Yellow vs. Green vs. Red Curry
Yellow Curry began to include turmeric as one of the vital ingredients, giving the curry its typical golden-yellow hue, as in traditional Indian curry. In fact, depending on the region, some Thai yellow curries resemble Indian curry while remaining distinctly Thai in taste.
Green Curry has been made more green over the years with the addition of fresh coriander (cilantro), lime leaf and basil. Of the three, green curry may be the hardest to distinguish, as the sauce isn't really green in color (see this photo: Thai Green Curry Chicken). Although all three colors may be spicy-hot depending on the chef, normally green is the mildest and red the hottest with yellow falling somewhere in between.
Red Curry has more or less remained the same, with traditional Thai chefs adding up to 20 red chilies to make it red and fiery hot. However, some modern chefs prefer to reduce the number of chilies in exchange for chili powder, which has the added benefit of enhancing the red color of this curry as well as giving it a deeper flavor.
To visually distinguish between these three types of Thai curries with an example of the ingredients used in each one, see these examples:
Other Types of Thai Curry
There are numerous other types of Thai curry in addition to red, yellow and green.
These include Thai Massaman Curry, which is a type of yellow curry with Indian characteristics, and Thai Penang Red Curry Chicken Recipe, a red curry with influences from Malaysia as well as other neighboring countries.