Staining. Not draining :)

Marble is very pourous.
I assume your exhaust fan is clearing out oil and grease from your cooking.
I found the following on the internet:
Favoredstoneguides.com
Oil and grease are some of the most difficult stains to remove from marble. As far as these types of stains go, it’s best to just avoid them altogether. Not only are they difficult to remove but they can also be difficult to prevent. If you must use oil or grease in your kitchen, be sure to wipe it up immediately.
To remove oil and grease from marble:
First, blot the stain with a clean white cloth.
Then, mix together equal parts dish soap and warm water and apply it to the stain.
Let it sit for a few minutes before blotting it again with a clean cloth.
Repeat this process until the stain is gone.

How To Prevent Stains From Marble.
Fortunately, there are some easy ways to prevent these stains from happening. For example, you can:
Use coasters for all drinks, especially ones that contain caffeine or alcohol.
Wipe up spills immediately.
Be careful with dye-based products like furniture or clothing.
Keep makeup away from your marble countertops.
Use mats or trivets for hot dishes.
Avoid using harsh chemicals or cleaners on your marble surfaces.
Seal your marble regularly.
Keep your marble surfaces clean at all times.
Never use abrasive materials to clean your marble.
Avoid leaving metal objects on your marble for extended periods of time.

I hope this helps you.

Has nothing to do with porosity. As installed with the oven’s exhaust right up against the backsplash, that will happen with any backsplash material, except soapstone.

At the very least, you need a back guard.

In reality, the GC should’ve installed steel studs, with a fireproof wall board in front, in the wall behind the range.

Look at the instructions that came with the range. There should be information about how to safely install the range (particularly the exhaust), in front of the backsplash.

Yep ^^^^^, scorching from no backguard.

I've seen this primarily with Thermador ranges.

I hope I'm not dragging this discussion into the weeds, but it looks like your range is set back into the backsplash, rather than having the backsplash behind the range, at least down to countertop level. I have not seen this before. Is this done often? I would imagine it would magnify the likelihood of scorching as the vent is even closer to the backsplash than it is in typical applications.

Put a piece of stainless steel behind the cooktop, ideally running up the wall to the hood and you should be fine.

Looks like your range is niched into the wall slightly. This is odd. We had marble behind a Blue Star and never had scorch marks. Can you try pulling the oven forward an inch?

Thank you all. Valuable advice. I think it has to be pulled forward - it most certainly seems pushed back too far. Also, I’ll check with the GC and the shop to see if there was a metal piece that came with it.
This is a Thermadore 48 gas range

The vents are used to exhaust hot air, otherwise the oven would heat the range body up too much. Low vents are called “island trim”. PA48WLBG is the 7” backguard, that you’d normally use if range is against the wall. However, from pictures it vents out the top, so hot exhaust will still be emitted right at the marble, just 7” higher. I think if pulling the range forward doesn’t help, you might have to live with the staining or have a piece of stainless steel glued to the marble, say from range to where the staining stops.

" Also, I’ll check with the GC and the shop to see if there was a metal piece that came with it. "

Backguard is available, not a product that comes automatically. But you are seeing why is it used first hand. Compounding the issue is as noted above, the range is inset into the backsplash, which should never have happened (did not even notice that when I replied yesterday). Pull the range aout, purchase and install the backguard and move on. I would not use a piece of stainless steel sheet as the lack of though process here would make me question if the area behind the backsplash was fire resistant.

Oh man, who authorized installing the backsplash over the vents? Yikes. Pull forward, install back guard . Don’t feel bad- all the ads show it without the back guard and that’s how every showroom has it. My number one complaint with showrooms.

Natural stone that has been resined, or has a sealer, can also scorch from heat. So can porous tiles like crackle tile, cement tiles, and even porous zellige tiles. All of those need to be sealed, and the sealer doesn't like being caramelized, and turns, well, caramel.

Thank you all.
I wrote Thermadore but this is Monogram!

I’ve spoken to the shop and they only suggested a 12 inch or full back guard. They said that even if we pull the range forward a bit, it would still have the same problem. Regardless, I’ll ask the contractor to pull it forward. The guard will really change the design of the kitchen, which is disappointing.

I’m surprised that more people don’t have this issue. Perhaps they don’t use the professional ranges much…

Yes the back guard is required when it goes against the wall. Pretty sure it even says so in the instructions…

The backguard on a pro style range is required when it's against the wall. Island trim is for island installs. Most have a 12" horizontal clearance requirement.

Wow, nobody ever mentioned this to us. I wonder why they don’t sell the piece at time of purchase. Could have avoided this unfortunate situation.

The wall backguard will also be less likely to send spills down into the range interior.

Pull it forward and install the backguard. Marble may be non combustible, but the wall behind it is not unless it was replaced with a steel stud wall with cement board, or is a block wall.

Another designer, not material or installer, error. Appliance and fillers should have been pulled forward.

If you cover it with stainless, you'll still get the brown, but it will be much easier to remove.