Simply using ice cubes dyed into primary colours with food colouring and left to melt until they combine and make an exciting new colour! (I'm sure it's a lot more exciting than it sounds when you're a toddler compared to an adult.)
We got Reuben to name off all the colours and explained to him that they will make new colours but I'm not sure how much of it he got before the colours actually started changing!
And as for calling them primary colours, that just flew over his head but I'm sure when we do this again when he's a bit older he will maybe understand them a bit better.
He did get excited about the colours changing. I left them on the window ledge so the sun could melt them quicker and he was constantly trying to climb up and shouting 'colour game! colour game!' so I think he liked it!
We encouraged him to talk abotut he colour and what colours he thought they'd make and asked if he saw any new colours appearing. His favourite was the green but I think that was just the most obvious one that had changed.