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:
$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.