Disclaimer: For Pony: add weight to "blue skin" in the 1st Form.

For Illustrious: Nothing it works without issues.

(There is also Additional Info at the bottom on how to switch between forms easier)

Phantylia

Weight: 1.0

Trigger: Phantylia

1st Form

Appearance:

Phantylia, Phan_1st, black hair, blue skin, yellow eyes, blue lips, gold headpiece, black gloves, long hair, jewelry, hair stick, fingernails, bead necklace, forehead ornament,

1st Form Outfit:

gold dress, gold bra, black dress, gold trim, green skirt, halter dress, long skirt, detached sleeves, black sleeves, halterneck, navel cutout, cleavage cutout, side cutout, 

2nd Form

Appearance:

Phantylia, Phan_2nd, purple hair, purple skin, black hair, blue eyes, purple lips, black headpiece, purple gloves, long hair, jewelry, hair stick, fingernails, bead necklace, forehead ornament,

2nd Form Outfit:

purple dress, black dress, purple skirt, halter dress, long skirt, detached sleeves, black sleeves, halterneck, navel cutout, cleavage cutout, side cutout, 

3rd Form

Appearance:

Phantylia, Phan_3rd, white hair, pale skin, orange eyes, red lips, orange headpiece, orange gloves, long hair, jewelry, hair stick, fingernails, bead necklace, forehead ornament,

3rd Form Outfit:

black dress, black skirt, halter dress, long skirt, detached sleeves, black sleeves, halterneck, navel cutout, cleavage cutout, side cutout, 

Additional Info: I sorted the tags in a linear way, so it's easier to switch between the forms.

For Appearance switch: Number Trigger Tag / Hair Color / Skin Color / Eye Color / Lip Color / Headpiece Color / Gloves Color (or basically just every tag before "long hair")

For outfit switch: Dress Color / Skirt Color (Or just overall every tag before "halter dress")

Everything else is the same, so you don't have to touch those parts of the Prompt.

The results of your generations are dependent on the additional LoRAs, weights, and models you use, so it may not work or come out as consistent as my sample images.