Game Boy Advance - Silicone - Purple
The Game Boy Advance Silicone membranes
Now i would strongly suggest that if you want the "Original" feel of the action buttons (A & B also the Dpad) is to use original silicone's and just use these coloured Start/Select buttons. This is fine to do in a solid colour shell
If your build is a transparent one then you have a choice between "original" feel buttons or the coloured look.