brainrender was first released with an accompanying preprint. While a lot has changed for brainrender since then, the preprint is still the best way to reference brainrender's contribution whenever you use the software in your work.
Please cite the preprint as:
Brainrender. A python based software for visualisation of neuroanatomical and morphological data. Federico Claudi, Adam L. Tyson, Tiago Branco. bioRxiv 2020.02.23.961748; doi: https://doi.org/10.1101/2020.02.23.961748