Hi there,
click into the cell you wish to colour and then on the top right hand corner of the screen, there's a little picture of a painpot (mext to the capital 'A'). If you click on the little arrow down button next to the paint pot, you chose the colour you want to fill the cell with. Next, to actually fill the cell with the colour, click on the actual icon of the Paintpot, and hey presto!