Looks good, here are some test cases off the top of my head.

guess: "SASSY", target: "BRASS", expect either "YYBGB" or "BYYGB". I believe your algorithm would return "YYYGB".

guess: "BRASS", target: "SASSY", expect "BBYGY"

guess: "ASSET", target: "BRASS", expect "YYYBB"

guess: "BRASS", target: "ASSET", expect "BBYYY"