If your collection uses dot notation, get it with MongoDB::selectCollection :
$colors = $db->selectCollection('my.colors');
Getting a collection has the same syntax as getting a database:
<?php
$connection = new MongoClient();
$db = $connection->baz;
// select a collection:
$collection = $db->foobar;
// or, directly selecting a database and collection:
$collection = $connection->baz->foobar;
?>
A collection is analogous to a table (if you are familiar with relational databases).
The API documentation on the MongoCollection class contains more information about collection objects.
If your collection uses dot notation, get it with MongoDB::selectCollection :
$colors = $db->selectCollection('my.colors');