In some cases, users may wish to use more advanced selections than are currently possible with ProteinVR’s built-in atom-selection system. Future versions of ProteinVR will enable such selections from within the ProteinVR interface itself. But in the meantime, dedicated users can "hack" the URL. Here’s an example where I’ve rendered the CA atom of only residue 57 as a red sphere.
Here’s a more complex example, where two separate atoms are highlighted.
Details re. this more advanced selection notation can be found here: https://3dmol.csb.pitt.edu/doc/types.html#AtomSpec