Aji amarillo is a yellow-hued chile pepper used in many traditional Peruvian dishes. It is readily available online and in Latin America markets, but in a pinch, there are some other peppers that make fine stand-ins for the tasty aji amarillo. Habanero and Scotch Bonnets work well, as they bring the heat and fruity undertones to whatever dishes they brush up against. If that's a little too much heat for your taste, I like to suggest jalapenos or even banana peppers. These are widely available and work as good subs for the aji amarillo. If you don't want any heat at all, go with the yellow bell pepper, and call it good.