PaintHalf: there are six colours, a random selection
is made for two colours. They have a half on them.
The paintbrush is controlled by the mouse click on it
and drag it to one of the colours on the mouse being
released. It will change to that colour. Drag the paint
bush to any of the square on mouse release the
square changes to that colour. The object is to paint
half of the squares in each of the two chosen colours.