You have two selectors here, one for selecting a fill color and one for selecting a fill pattern. In the fill pattern, there's an option that specifies a transparent pattern. This is the upper left pattern that looks like two empty boxes. When you select a color, filemaker automatically selects the solid fill pattern for you.
You can set the default fill color and pattern by entering layout mode and making these selections while no object is selected. Then each new object on that layout will recieve the color and pattern settings you specified earlier.
On the mac, in layout mode hold the command key and click on a field that you want the format of. Now all new fields will use those fields settings as the default.
Alternatively, you can select outside of any field (eg click on a blank area of the layout) then set the colors, etc in the formating pallete you wish to use. All new fields should take on those settings.
To change the fields you already have click on a field that is working well in layout mode, then choose the little paintbrush (format painter) from the toolbar. Then click on your messed up field. This will take the formatting from your good field and paint it on to your messed up one.
Thanks SOOOOOO much, sorted. I didn't imagine it would be in the pattern section (might have spotted it if tooltips were there).