With acrylics, which come in pots, bottles or tubes, it's easy to know exactly which colour's inside. With watercolours, it can be more difficult. That is if you're using the pans and want to know the exact names of your colour. With the higher quality paints, the names of the colours is usually found not only on the wrapping, but also printed on the side of the pan. But of course once you've put them all into your box, you can't see them anymore. So in order to know what's in your paintbox, you'll have to make a list of them. And if you're a painter, and like things to be visual and colourful, rather than having a dull list, the obvious thing is to just paint your paintbox and label all the colours.
Et voilà, your own handy little reference sheet with all your colours in your paintbox.