Hanuman Sindoor is a traditional orange colored powder, usually offered to Lord Hanuman idols.
Lord Hanuman is the symbol of PRAANAA so that praan shakti is symbolized in the red or (sindoori colour). Hanuman being symbol of Power and faithfulness, has been shown with application of Sindoor on his body. In Hath yoga branch, Sindoor/Mercury is used in Al chemistry to make 'Gold'. This is very secret process and may be very few in this world know.
As per Ramayana, Mata Sita was applying Sindoor, when Hanumanji came and asked Her why She was doing it. She explained that it was for the longevity of Sri Ram- the more, she applies, the longer He will live. Upon hearing this, Hanumaji went and covered himself in sindoor, and this is the reason why His idols are often covered in sindoor.
Proper use of Sindoor or Mercury for that count, gives power to fly in Air, extra ordinary physical strength, change himself in to desired form etc. All these qualities were connected with Lord Hanuman.