The difficulty with using chili powder lies in replicating the distinctive color and flavour characteristic of Massaman curry paste. Additionally, mild chili powder often includes a blend of spices, resulting in a different taste profile. If you choose to experiment with a substitution, a general guideline is to use 1 teaspoon of powder for each whole chili. However, it's crucial to avoid using hot chili powder since Massaman curry isn't intended to be overly spicy, as it utilizes dried chilies with the seeds removed. The use of a spicier powder may risk making the dish too intense and potentially inedible.