How can I save a point cloud that is segmented. It has x,y,z values and a class value.

### Checklist

- [X] I have searched for [similar issues](https://github.com/isl-org/Open3D/issues).
- [X] For Python issues, I have tested with the [latest development wheel](http://www.open3d.org/docs/latest/getting_started.html#development-version-pip).
- [X] I have checked the [release documentation](http://www.open3d.org/docs/release/) and the [latest documentation](http://www.open3d.org/docs/latest/) (for `master` branch).

### My Question

Hello,

I am trying to save my point clouds, that I am later on going to use as segmented point clouds for my neural network. I have a list of points and each point has [x, y, z] values and a class value. How can I save a file of this format using open3d? I thought of using [x, y, z, r, g, b] since I have only two classes but couldn't get it to work. Any help would be appreciated!