Compound shapes have different parts which can be subselected to apply fill colours. If you click on the shape, pay attention to the selection handles, then click again and see if the handles indicate that a subshape is now selected. Try clicking on the subshape until the one you want to change is selected and apply your preferred fill.

Once you know how to effect the change, make the stencil editable and edit the symbol directly. Changing an instance already on the page won't change the stencil.

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Andrew Lockton
Chrysalis Design, Melbourne Australia