Yellow to orange

Dr. Brown suspects that you have confused the Biuret test with the Xanthoproteic test, which also tests for proteins. Indeed, a positive Xanthoproteic test is indicated by a color change from yellow to orange. 

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.