Panniers, to match or not to match

[mjr]

I'd put the black one on the non drive side and pick something in bright colours and/or retro reflective for the drive side (to make your bike silhouette more visible to those behind).

Adding a bright patch to a silhouette will disrupt the outline and make it less visible, not more. Pick the colour you like because over a quarter of drivers can't see properly so it ain't gonna stop them claiming you leapt out from nowhere! Might as well enjoy looking at it instead.

And I often ride with different panniers, if that's the best way to carry what I need.