The post ‘Duality is mithyA’ (see here) is based on the verses 19.20 -22 of Upadesa Sahasri (US). Swami Parmarthananda has discussed (transcripts of his talks on Chapter 16 of US) the same topic with a different reasoning which according to him has scriptural support though he has not cited it. 
Advaita Vedanta acknowledges six means (Pramana) of knowledge of which direct perception and inference are important. I see red colour and green colour and say that the two colours are different which is my experience too. If asked whether I see the ‘difference’ as an object, my answer is in negative. That is to say, there is no direct perception of the ‘difference’. It also means that the ‘difference’ cannot be inferred, because for inference, there should be a previous direct perception. Other means of knowledge, namely, comparison, postulation, non-cognition also do not prove difference. Sabda Pramana i.e., scriptures, affirm that ‘difference’ is not real though it is experienced. QED