Polychromos Swatching

So I’ve done a huge swatch of those Polychromos coloured pencils and here it is.  They look good.  Even the cobalt green and rose carmine will end up getting used at some point.  Looking at the while set, I’m wondering whether there are any areas where I’m short of colours and could expand the set – after all I have 36 colours and a case with 72 slots.  And it’s hard to find holes.  Maybe at a stretch I could do with another yellow and another brown?  Or some more flesh tones if I go down the portrait or figure drawing route?  Or maybe if the set is so well balanced, I could only expand with a well balanced collection of 12-24 more colours?