Why cant you say "що твій улюблений колір?
Use "Що" if you are expecting the answer to be a noun, use "Як" if you are expecting the answer to be an adjective.
Як wouldn't work here...
Is there anyone who can explain when "Як" does and when it does not require being changed to "Який"?
Як would mostly translate as How, and Який would be What/which one (musculine, also Яка, Яке, Які)