So in this world I was writing about I need one exclusive zone of the planet to be purple and their plants to be adapted to this atmosphere.
What would I need?
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To get a purple sky you need either:
What do plants look like that must survive on purple light?
Purple lights are used by indoor plant growing setups all the time, the one I linked to there is advertised at 70% efficiency gain over white light for the same amount of final crop.
This region has airborne bacteria in such quantities that the light filtering thru them is purple.
Start with autotrophic bacteria which are purple from their photosynthetic pigments. Here are some good ones!
These purple organisms generate intracellular hydrogen, as I proposed in this idea: Which conditions would make aerial filter feeding successful?
They hydrogen makes them neutrally buoyant. High relative humidity keeps them from dessicating and they can fix their own nitrogen. They live in the air all the way up to where atmospheric pressure is so low they dry out.
Beneath the purple haze, it is dim and ... purple. Also, dead organisms fall from the sky periodically. They are still purple when dead, and so things on the land beneath gradually become purple as well.
Frame challenge. It's all in the eye of the beholder.
You are on an alien planet. Therefore I assume you have aliens with ... alien physiology.
Light works the same way but your aliens eyes are designed to view a different wavelength of the light spectrum. So their world looks purple to them, while to us regular humans their world looks more blue, or green, or dusty red.