Solid Fills include
•the Foreground Fill: You see only the Foreground, because it covers up the Background.
•the Background Fill: You see only the Background, because the Foreground is set to zero.
•Three fills whose foregrounds are of predefined Opacity (25%, 50%, 75%).
These percentages can be adjusted by hand in Options > Element Attributes > Fills, by entering a new percentage value in the Opacity field.
In the Fills Dialog Box (Options > Attributes > Fills), click New, and choose Solid fill from the New Fill dialog box.
In the Fill Appearance panel, enter an Opacity percentage (or use the slider) - in this case, 65. The new Fill Type will henceforth appear in the fill pop-up.
You can create a new solid fill type with any translucence value you wish.
View the result in the Preview window.