It's dark and grey this time of year, that's why we cheer ourselves up with lovely sparkly tinsel and colourful baubles. But if that isn't enough for you, here's another chance to bask in some glorious colour.

Maths behind the rainbow

The only good thing about a wash-out summer is that you get to see lots of rainbows. Keats complained that a mathematical explanation of these marvels of nature robs them of their magic, conquering "all mysteries by rule and line". But rainbow geometry is just as elegant as the rainbows themselves.

Colouring by numbers

How does a computer understand the colours to be displayed on the monitor's screen? It's all about red, green and blue and numbers written in a special way.