Three different-colored Thai curry pastes that are readily available (plus Panang curry), green,yellow, and red. How do they differ in taste and heat? I’m pretty sure that it’s not green for “mild,” yellow for “medium,” and red for “scorch your mouth.” As I age, my tolerance for spicy foods is, alas, diminishing.
Thai curry pastes can have a variety of ingredients, depending on the style and region. Some of the most common components, however, are shallots, garlic, chiles, lemongrass, galangal, fermented fish/shrimp. The chiles give the color: a red paste is made with dark red (dried) chiles. Green is made with fresh green chiles, and yellow with light red (dried) chiles. The only other pigmented ingredient in curry pastes is fresh turmeric, which gives a brighter color, and makes the paste yellow when combined with bright red chiles.
Green is usually the hottest.
I'm not Thai but I made most of the pastes myself a couple times by now, trying to answer to the best of my ability.
Green is made with fresh green chilies, all others with dried red chilies (and added fresh red in a few cases). By convention, storebought pastes tend to be on the hot side for green and red pastes, milder for panang and massaman, yellow in between. If you make your own, you can adjust the heat level by the mix of larger, milder and smaller, hotter chilies you use.
Yellow is a red paste with added spices that you would find in south indian or anglo indian food (some recipes simply add the anglo-indian invention of curry powder - which, though it is used in the more northern-inspired anglo-indian cuisine, is more like a south indian sambhar powder than the effective mix of "darker" aromatic spices in a north indian dish. Fenugreek or mustard powder, as found in many curry powders, seems to work well here). Some recipes call for ginger in addition to galangal. Some recipes suggest roasting almost all the ingredients before making the paste.
Massaman is also based on a red paste, with the addition of a more north indian set of spices (more emphasis on cloves, black cardamom, cinnamon, cumin... Hold the fenugreek and mustard!).
Jungle (+fresh peppers), panang (has or needs ground peanuts added) and gaeng kua (MINUS cumin/coriander/black or white pepper) are also slightly altered red pastes.
Khao Soi paste is a completely different mixture, somewhat resembling yellow paste but with some things removed.
Adding fresh or ground turmeric seems to be standard in yellow and khao soi, and negotiable in the other pastes (though recipes that identify as southern Thai usually do have it added.). It is worth trying out, since it really enhances the color even of the non-yellow varieties.
Omitting the soup pastes here since it is still unclear to me whether "Tom" soups should ever have any curry pastes added or just whole aromatics and shrimp and/or nam phrik paste.