Yellow to blue-black

Dr. Brown suspects that you have confused the Biuret test with the iodine test, which tests for starch. Indeed, a positive iodine test is indicated by a color change from yellow to blue-black. 

Dr. Brown suggests that you revise the reagents used in the Biuret test, including their color, and the chemical reactions involved so that you can refresh your knowledge before you carry out this important test. 

Please go back and reconsider.