It's simple, click the grid cells to launch a pallete for that cell. Select a colour and click 'choose' to paint them.
We recommend using your browsers zoom in/out feature to help with making your selection.
The quickest way to zoom your browser is to press [Ctrl] + + (Mac users can use [Cmd] + +) to zoom in and [Ctrl] + - (Mac users can use [Cmd] + -) to zoom out.
The quickest way to return your browser to the original zoom setting is to press [Ctrl] + 0 (Mac users can use [Cmd] + 0).
Additional browser-specific instructions can be found here.