I think the far left one makes it match the color of your phone, which is pretty neat. Mine makes my icons yellow, I have a yellow 14 PlusFound something new. If you customize home screen icons, now there's two new buttons (the two on the left), haven't quite figured out what they do, but i noticed the one on the left changes the tinted icons a bit.
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Update: I think the button all the way in the left makes the icons tinted and more translucent /glassy. I like the effect.