here is a example showing how to use cell masks

go to the toolbox and click the purple rectangle

select blue from the color box and draw a blue rectangle

select purple from the color box and draw a purple rectangle

go to table->edit cell masks

unchecked the checkbox that says "bit 2 (4)" then click OK

select yellow from the color box

draw a rectangle on top of both the blue rectangle and the purple rectangle

here is a picture of it.