When you go to cell format, select cell shading and the colour (green) from the colour pallete.
Then click the pattern drop-down.
A second colour palette opens, as well as the pattern selector.
The Pattern selector closes.
Then select the Pattern drop-down again, and select the pattern required.
The pattern should now be white on a green background.
Does this sequence work?
If not let us know what happens when you follow this routine.