Grey lenses keep colour neutral. Brown and copper lenses push contrast, which is why guides and surfers reach for them: the edge between a swell line and the flat water behind it gets sharper.
Tidal uses a copper base with a slight rose bias. On water it separates the surface from what sits under it; on a trail it lifts the shadow side of a rock so you see the step before you take it.
If you mostly drive and walk, choose grey. If you spend your days looking at water or dirt, choose copper.