Did some digging and couldn't find a definitive answer...were the rear wheel wells body color or painted black from the factory? After doing some cleaning and removing years of road grime, it kind of looks like they were black in some areas and body color in others.

 

it kind of depends, Mazda assembles the body, minus the bumpers, and doors, paints it. then on light colored cars, white, red, etc they will spray the wheel well with some black, seems to be by hand cause its inconsistent.
on dark color cars they don't need to bother. basically the idea is that you want to look at the car and not see the wheel wells.

also plenty of dealers sold various undercoating and rust proofing.

best course of action is to look at your car, and maybe others in the same color-ish.

 

I've always wondered about that too. I have blotches of red amidst mostly black in the wheel wells of my VR FD - and Miata. Always contemplated painting the area black. Maybe with some undercoating type product...

 

^ Same. It appears they are coated with a rubberized black undercoating material on my VR car, but there are areas of red poking through in spots where the layer is thinner.

If you're going for a correct restoration, putting a flat (not glossy) black undercoating over the red painted surface should do it.

 

It may have just been my imagination but some years ago I sprayed additional rubberized undercoating on the upper rear wells. I thought it helped a little with road noise.

 

Mine are super splotchy. Wasn't sure if I should go all black or just clean and coat what's already there.

 

If you’re going for the OEM look, I’d leave them as they are.

 

Spray some degreaser on there and go to town. They're actually body color under there.

 

Looks typical.

Unless the reinforcements are removed for painting or going concours original, if you were that keen, might have been worthwhile to have installed a later subframe while down there.

With some evidence of mud spray in usually protected areas - probably worthwhile investing in a replacement liner if it's missing or broken......assuming it's not NLA yet!