Not possible to edit shapes after insert them into table

Edit your shape before you copy/paste into a table cell as the Format menu : Shapes and Lines sub-menu does not offer any editing capability afterward.

The workaround is that you can select the shape in the table cell, and then from the right-click menu, choose cut to remove it from the cell, and then paste it elsewhere on the same page outside of the table. Now, you can edit the shape, and then cut and paste back into the cell again.

Pages 8.1.

What I have already stated. No other workaround because once the shape is within the table cell, you no longer have any control over that shape's edits or style (color) changes.