Green chili is a catch-all designation that applies to several different varieties of green chili peppers, from the mild Anaheim to the smoky, medium-hot pasilla to the fiery serrano. Because of the variety to choose from, selecting a proper pepper substitution is simple. Taking the dish you are preparing into consideration, select a pepper whose flavor will complement the other ingredients, keeping in mind the spiciness so it isn't overwhelming.
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If mild green chilies are more your speed than their fiery counterparts, go with Anaheim, banana or poblano peppers. Try using a pasilla, or a green fresno -- red fresnos are hotter -- if you'd like to add a noticeable kick of heat to your dish, but not overwhelm it with too much spice. A jalapeno's hotness can vary from medium-hot to hot, so taste test before incorporating it into your dish. And if you like it hot, heat things up with a serrano or Thai chili pepper.
If the dish you're making doesn't depend on the appearance or texture of green chili peppers, various spices can be substituted in a pinch. Try substituting chili powder, cayenne pepper or crushed red pepper for green chilies. Begin by adding just a few dashes of these potent spices, tasting and adding more spice until you are satisfied with the flavor and heat level.