I've posted in the past about how when I get the playdough out I can't bear to mix up the colours, and so one colour must be put away before another comes out. This suits me just fine, but it must be quite boring for Harry. So I've found a way to let him have the colour mixing experience without making me start to twitch!
Simply take a plain shortbread biscuit dough (here is my simple shortbread recipe), divide into sections and use food colouring to dye each section a different colour. You might find that the dye makes the mixture a little sticky, so you will need to add a bit more flour. Alternatively if you can get hold of some gel food colouring you will have much more vibrant colours from just a very small amount of colouring.
Then present the dough to the toddler and let him happily squish the different colours together. I made base biscuit circles out of one colour and showed Harry how to make spots in different colours, but I think that now he is old enough that he would have been quite happy without any guidance. You can make all sorts of designs - flowers, smiley faces, stripes, spots and so on. If your child has a particular interest, for example television characters, you could even have an attempt at recreating them.
Just a word of warning, this is an activity which is best prepared by an adult in advance, unless you want to complement your multicoloured biscuits with a multicoloured child and multicoloured kitchen.