Was the (light blue) Cure Beat and Cure Symphony scan◊ fake from the start? It seems as though one way or another, Waon and Seika were intended to become the Sixth Ranger Cures, seeing as they were relatively prominent secondary characters. Not to mention the fact that the Cure Module actually has light blue and purple gems.◊ Siren got a pass, but were they ever going to explain how Cure Muse uses a purple Fairy tone to become a yellow cure? note Granted, she is a princess. Some sort of acknowledgement or Lampshade Hanging would have been nice, though.
The scan was fake. It's always been fake. The production staff "leaked" it to draw suspicion away from Ako and Ellen as the actual Sixth Rangers, after a similar production scan last year leaked Sunshine and Moonlight's identities before the first episode had aired. Waon and Seika were never "meant" to become Cures: they were always Red Herrings. As for the Muse thing, that's never been clearly explained. My personal theory is that yellow and violet are complementary colors, so they just rolled with that.
The gems on the Cure Modules represent the Fairy Tones used to transform. It is a little odd that Ako uses a purple Fairy Tone to become a yellow Cure (especially considering how all the others have matching colors).