Hello ! I found at least one way to get all the colors in the 20th anniversary.
I gave all I know in this topic https://www.tamatalk.com/IB/topic/194510-color-changing-items-for-the-various-mixs/
Hi ! Here the color changing foods I found for the 20th Anniversary mix :
Red : Watermelon juice (Suika jūsu) 130₲ (Snack at Restaurant)
Tomato ramen (Tomato rāmen) 110₲ (Meal at Restaurant)
Minestrone (Minesutorōne) 160₲ (Meal at Restaurant)
Peach (Momo) 100₲ (Snack at Makkakka Town)...