I was looking at the video Maangchi has up on making KimChi and a viewer had asked a question on the suitability of Chili powder as a substitute for Red Pepper Powder or Flakes. I can answer that question. Absolutely not.
Chili powder is a mix of many different spices, and is not even similar to Red Pepper flakes. The closest thing we have in North America would be crushed chilis, which would then have to be ground into powder (or Cayenne Pepper) and either of these would lose a pungency that exists in the Red Pepper Flakes or Powder of Korean variety. Both ground crushed chilis and cayenne pepper make a reasonable substitute if the proper flakes/powder are not available, but both flavour and the red color would suffer. I hope this helps someone to not ruin a batch of KimChi with Chili Powder.
Oh, by the way. Excellent video and excellent recipe for KimChi, Maangchi, but you knew that, right?