Keynote My Shapes

No problem, custom shapes are vector objects so they that can be freely edited:

• place the custom shape on the slide
• with the custom shape selected, go to: Inspector / Format / Style and use any of the tools to change colour

The shape form can be changed; right click the shape and select: Make Editable then drag any of the red handles to change the shape.

Click on the Shapes button in the tool bar, go down the list to the last entry; My Shapes:

You need to save as a new shape:

Right click the changed shape on the slide and select: Save to my Shapes