Control of the Show Fill Color Legend checkbox on this page is shared with the controls on the Legend Display Options page of the dialog. The next option on this page includes a checkbox that will Revert legend to fill color variable values. This is simply a way of reverting the text of the Fill Color legend back to its defaults.
 
The other controls presented on this page will depend on how Symbol Fill Color is defined - specifically if Symbol Fill Color is defined by a continuous variable or not.

Symbol Fill Color not defined by a continuous variable

If Symbol Fill Color is set to either “One color for the entire graph” or is defined by a categorical variable, the only remaining option on this page will be a checkbox to Reverse legend order.
 

Symbol Fill Color defined by a continuous variable

If Symbol Fill Color is defined by a continuous variable, then this page will also have controls to customize the appearance of the color scale legend. The Orientation dropdown will allow you to specify if the color scale legend should be vertical or horizontal. The Border options allow you to specify the border thickness and color with the appropriate dropdown menus.

The Scale section of these options provides a means for defining the Interval for numbering on the color scale legend, and the value at which the color scale legend should start. There is also a checkbox to Reverse legend direction in this section.

Using the options in the Location of numbers/labels section, the Position, Direction, Tick length, and Tick direction can be controlled.

Finally, in the Format of number/labels section, a custom Prefix or Suffix can be added to the legend labels, while the number Format, amount of Decimals, and Thousands style can also be specified.