Yellow stains on Indian stone

If they left it in a mess and you weren't happy you should not have paid them until they sorted it out. You stated that they had left a mess all over it and told you to clean it with brick acid.

If the slabs were layed on four/five spot bed of sand and cement, instead of a bed of cement. Then water will get into the void, which will then cause the slabs to stain.

When you get rust spots it's more than likely its a mortar mix than cement, and you don't use brick acid on Indian stone, only solution is to take it up and start again,

First you need to know if its mortar stains. If it is you could get a mortar stain remover and test it on a small area to see if it affects the flags then ask the people who did the job to come back and clean it. You have to be careful that any stain remover does not affect the stone or use a soft wire brush with soapy water. That should remove any mortar stains.

Hi as the Indian sand stone is a natural slab you may get a natural phanonama coming out the slabs like rust or white which is floressance - you don’t want to seal it but there may be a solution for cleaning them Indian sand stone should be laid on a full bed of mortar mix and pointed with the same cement you are using not the resin you buy in the tubs 👍