In a lab, chemicals are identified by a colour coding system. There are 21 different chemicals. Not more than one chemical is left colourless and not more than one chemical is coloured with all the colours available. Rest of the chemicals are coded with either 2 colour-pair (unique) or 3 colour-pair (unique)· lf the order of colours in the pairs does not matter, what is the minimum number of different colours needed to code all 21 chemicals