Look at the following description and find out the relevant word from the passage:
Substance that is added in small quantity:- a___________.
In the line "Various additives such as coffee decoction, tea... are mixed with the henna powder" the word 'additive' means 'a substance added to something in small quantities to improve or preserve it'.
This meaning is the same as the given description: substance that is added in small quantity.
Hence the correct answer is: additives.