I read that we need to wait at the purple curb for the shuttle. Where is located this location in the LAX airport ? thanks

Depending on what airlines you are coming from after you retrieve your luggage proceed outside and go to the center divider and you'll notice that they are a couple different colors you may have to go 30 ft or 40 ft one way or the other and you will be at the purple curb and they will be there within 15 minutes

On lax airport outside

There will be multiple car hire firms listed on the purple pillar. Just wait for the black sixt bus that will stop next to the purple section.

Just front of the door. You will see that color above of the people waiting the shuttle when you go outside.

At LAX you wait in the middle curb where the purple sign is at the post. Just outside each terminal