WHEN I AM ASKED MY COMPLEXION, CAN I REPLY AS : YELLOW?

In order to answer this question, we have to know the meaning of "Complexion" first.
According to Merriam-Webster, complexion has the meanings of
1. attitudes and inclinations
2. the hue or appearance of the skin especially of the face
3. overall aspect or character
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If you use the second definition of complexion, that will mean you have a jaundiced face which indicates a liver disease.
So the definition of complexion is not exactly the same as the race you are, but the look on your face.

In my experience as a passenger while I was travelling by airliner,and prior to landing ;usually there would be a pamphlet of immigration was needed to complete by you. And that pamphlet had questionnaire about your complexion and in case your were Chinese and would require to write "beige" ? instead of yellow. Yip