First of all, curry is a blend of spices, not a single spice. It is primarily used in South Asian and Southeast Asian cooking although it is fast becoming popular outside of these regions. What spices are included in the mix varies from cook to cook and region to region.
For the home cook, the most common source of curry is the grocery where yellow curry is usually sold in powder form. During recent years, curry pastes have been making a persistent appearance and these pastes are either yellow, red or green.
What’s the difference? First, what’s with powdered curry and curry paste? Curry powder is more common in South Asia while curry paste is associated with Thai cooking. Consequently, although both are spicy, the spices in the mix can vary greatly. South Asian recipes will smell and taste more authentic is curry powder is used while Thai and Southeast Asian dishes will benefit more from curry paste.
Can they be used interchangeably? Well, if you’re not really particular about labeling your dish as South Asian or Southeast Asian, yes, I suppose so. I mean, so long as you get that bright color and spicy flavors, right? But if you want optimum results, and this is something I learned fairly recently having been raised on chicken curry cooked with generic curry powder, you have to choose which will go better with the rest of the ingredients in your dish. For instance, Thai curries which are often cooked with coconut milk taste so much better with curry paste. Indian vegetarian curries smell and look better when cooked with curry powder.
Now, what’s with red, green and yellow curry paste? How are they different from one another? The chilis which vary the amount of heat. Green curry paste has green chilis, red curry paste has red chilis and yellow curry paste has yellow chilis. The rest of the spices in the mix may or may not be the same depending on the maker.
For instance, green curry paste often contain cilantro, lemongrass and kaffir lime leaves. Yellow curry paste may include ginger, coriander seeds and lemongrass. Red curry paste may have more chili powder than fresh chilis.
Which is the hottest — green, red or yellow? Generally, very young green chilis are the hottest while yellow chilis are the mildest. That should lead to the conclusion that green curry paste is the hottest. But, it truth, it varies since there are so many varieties of chili. Bird’s eye chili, for instance, is hotter than finger chilis. So, a curry paste made with red bird’s eye chili will be hotter than a paste made with green finger chilis.
So, don’t think that curry is curry is curry. Read the labels, check what other spices and herbs are in the paste, and decide which blend appeals to you most.