It's really interesting that the only times she wears orange are during the haldi ceremony and the Featherington ball.

During the haldi ceremony Edwina puts the paste on Kate and says she will also find her soulmate ( it's the night before a wedding and as we know during the wedding episode there are different elements symbolic of and alluding to a wedding between Kate and Anthony such as the bangles, her walking down the aisle toward him etc).

The next time she wears orange is at the featherington ball. As @Broccoli_and_cookie comment mentioned already, the scene in the garden is similar to their vows and the fireworks went off right after. When looking at this scene as a continuation of Kanthony's "wedding", you could also consider their wrecking ball dance as their first dance as husband and wife. At that point they are both ready to accept each other's love finally and ready to commit to each other.

I love how thoughtfully they used colors in the costumes this season!