No sand paper!!!!!!!
Try dish soap and a tooth brush....and by dish soap I mean Palmolive....not Cascade.
They look like they might be a different stone...perhaps sandstone.
The yellow is part of the stone....get the dirt off and then just accept the color.....they are very lovely....worth painting to enhance their natural color.
Linda C

I'm assuming Linda means painting the WALLS to enhance the stone color, not painting the stone !!

Oxalic acid. Will make them as white as they can be.
Casey

Oxalic acid will seriously harm marble or any limestone...please don't use it to clean your panels.

And yes I meant paint the walls to match the stone! LOL!

Oxalic acid is used in the polishing and finishing of marble. Admittedly, if used improperly it will do damage, if used correctly they will be, as I stated, as white as they can possibly be.
Hint: don't soak overnight. Hint #2: apply sparingly and as soon as it is clean, rinse thoroughly.
The only reason I'm suggesting oxalic acid is because from the photo, it's clear that the grills do not have a highly-polished finish to begin with. It would not be appropriate to use acid on polished marble. You would have to use a peroxide treatment, but that only affects organic staining, and is ineffective on rust.

Casey

sherriz please come back and post your finished project - I'm very interested in seeing how you are going to use these. Those are beautiful pieces.

Thanks all. I will take these into a stone yard and ask them as well. Maybe I will test Oxalic Acid in one small spot. These won't be installed and photo ready for about 6-7 months. Will be back...

Oxalic acid powder is used in obtaining a high polish on marble surfaces. It's used as a polishing agent. I think it dissolves a small amount of the surface and allows the abrasive to bring up the surface to a high polish.
By using Oxalic acid...or any acid to clean these you are risking making the carvings less crisp.
I suggest asking a museum that deals with cleaning marble sculpture.

They look like limestone grills used in India or the mid-East, and those tend to be on the yellowish side. It's not dirt, it's the natural color of the stone.

I'd figure out a color scheme to use them "as is" if mild soap and water didn't clean them enough.

What lazygardens said. Contempory sandstone lattices imported from India.

Here is a link that might be useful: sandstone lattice importer

Ok,
so it's not marble anyway. So, scrub with comet w/bleach; you probably can't hurt 'em!
How big are they anyway? The pic gives no real sense of scale.
Casey

They are about 12" x 18" each. I also have a 12 x 36" one that I will put horizontally between these two. Different pattern.

How can I determine if they are sandstone vs. limestone. Thanks for all your help here.

Limestone is a sedimentary rock composed mostly of fossil bugs and shells....while sandstone is a sedimantary rock composed mostly of quartz bits.
Put a little vinegar on a corner and see if it bubbles.....that would be limestone.
most likely they are sandstone...because sandstone can be harder, stronger and last longer than limestone.....but then you have The Alhambra....which has, If I remember correctly lots and lots of stone "lace" made from marble...