Hi bet-beach,

Here is an idea: to take the Vertex (Vertex Color Give) information from a small portion (Vertex. Color Share) and provide it back with a "blur" to a new one (Vertex Color Get).

The Broom (Vertex Color Share) has a lower Decay setting, to update its information as the broom (Vertex Color Share) moves, while the Vertex Color Get has a 100% decay setting, to keep the changes.

Ideally, the source would be separate, to follow a top-down process in the Object Manager, but I assume it works just fine here.
Example
https://projectfiles.maxon.net/Cineversity_Forum_Support/2025_PROJECTS_DRS/20250912_CV4_2025_drs_25_PAcm_41.c4d.zip

Screenshot 2025-09-12 at 11.50.55 AM.jpg

Enjoy