I apologize for this being very late (I'm pretty sure Toucanadian is mostly inactive now), but Castform's other forms finally have obtained shiny forms (that's a lot of forms)! They were first introduced in Pokémon Home, as transferring a shiny Castform to it allowed for the other shiny forms to be seen for the first time.
Also, just in case someone isn't sure, the one on the left is the shiny variant of the form.
Rain Castform has a darker cloud at its bottom, a teal main body, and green "goggles", instead of the usual white.
Sun Castform has a pinker cloud, a red main body, and "goggles" that change to be a faded red.
Hail Castform has a blue main body, with yellow "goggles."
In addition, it appears as if Rain and Hail's mouths become slightly paler.
I Hope I Helped!
Source 2 (The start of the video celebrates the new Shiny forms of Castform)