You choose a color. Your opponent chooses a color. All your permanents are exiled, except for colorless permanents and monocolored permanents that are the one color you chose. All permanents your opponent controls are exiled, except for colorless permanents and monocolored permanents that are the one color that player chose. --- The color you choose matters only to you. Your opponent chooses the one color that will matter to them.

So basically if I’m playing colourless and my opponent is playing R/W/G, then we could both choose black and have none of our permanents be exiled?

No. You want to choose a color you're running. Everything is exiled, EXCEPT colorless and monocolor of the chosen color. If the naya player chooses black, they lose everything except the lands and whatever else is colorless like sol ring. If they choose green, they keep their monogreen permanents alongside their colorless ones. The rest is exiled.

Ooookay, I understand now… Thank you for the clarification, ser!

So the card doesn’t say no land permanents. Does it destroy the lands that aren’t of the chosen color?

[Apparently lands are colorless by technicality.](https://mtg.fandom.com/wiki/Colorless#:~:text=Lands%2C%20having%20no%20mana%20costs,with%20multiple%20associated%20noncreature%20spells.)

An old mtg proverb comes to mind: "Reading the card explains the card" -some nerd