Red chilies are a central ingredient in Thai cuisine, but Thai cooks also use green and yellow chilies, hence the three famous colors of Thai curry: green curry, red curry, and yellow curry. Each Thai dish is usually made with a certain chili, although you can always easily substitute red chilies if you can't find green or yellow. Peppers of all sizes and shapes can be found in Thai cooking, including sweet bell peppers.
Thai Bird's Eye Chilies
Characteristic of the Thai sense of humor, the hottest of these chilies is called park kid noon, which literally means "mouse droppings." But they're also popularly known as Bird's Eye chilies, or Bird peppers, simply because looking at one from the stem-end can appear like the eye of a bird. When they are fully mature, they turn red. They are a variety of Capsicum frutescens and were originally the chili used in Sriracha sauce.
Heat Warning: Bird's eye chilies are very hot. On the Scoville scale, they rate between 50,000 and 100,000 Scoville units. That's over 20 times hotter than a jalapeno but three times less spicy than a habanero. The heat can sneak up on you as the mature red Bird's Eye chili can have a delayed potency, with the heat building as you eat the dish, and then lingering long after you stop eating. If your taste tends toward medium or mild-spiced foods, try de-seeding them, then mincing for less heat.
Caution When Preparing Bird's Eye Chilies: You may want to wear rubber gloves when working with these chilies, especially if you are a contact lens wearer. The chili oil and heat chemical, capsaicin, can stay on your fingers for several hours and really sting when you take out your lenses or touch your eyes.
You're in for a painful surprise if you didn't wear gloves. Also, the hot chili chemical (capsaicin) will stay on your contact lenses and sting you again the next day.
Where to Find Bird's Eye Chilies: Try looking at your local supermarket for these chilies. As Thai cuisine has become more popular, they are more widely available. If you don't find them there, most Asian food stores and markets sell them. They are available year-round.
Substitutions for Thai Chilies
If you can't find fresh bird's eye chilies, a good substitute is dried chili flakes, which are available in any supermarket spice section (also called crushed chili). These are generally made from cayenne peppers. You may be able to find whole dried cayenne peppers as well. They are about half as hot as Bird's Eye chilies, so take that into account. They blend with Thai dishes in the same way as Thai chilies.
If you can't find crushed or whole cayenne, you can use powdered cayenne pepper to spice up most Thai dishes. This form of child can give your dish a reddish coloring. That is often desirable, such as if you're cooking a Thai red curry, for example.
Serrano peppers can be substituted, but they are only about a quarter as hot as Thai peppers.
They can be a good alternative in Thai dishes that call for fresh peppers.