Installing double bowle sink

First time poster here. I am about to have a local company remove the existing drop in shower and add a second shower head in the ceiling, tile the walls and ceilings and install a new shower door. I went to the local tile shop and picked out the tile for the for the shower floor and Threshold. The company that will be doing the work has told me that I can't have a shower door sitting on tile and that it must be marble. They said that they can't guarantee the door will not leak if it is on tile. I really don't want a piece of marble for the thresh hold but the tile I picked out. Is my contractor correct? For some reason I think no because I am sure there are doors installed on tile that do not leak. Also, tile store said it can be done so I bought the tile then the owner said the door installation on tile is not recommended. Help please....

My kitchen has an old mustardy yellow tile countertop with dark brown grout. I hate it but I can't afford to replace it right now. I've heard that there is a paint which can be used on tile and will make it look like multi colored granite. Has any one ever tried this? How durable is it? If there are any other suggestions out there I would be open to them as well. Just looking for something affordable that I can do myself possibly over a weekend. Any advice would be greatly appreciated.:gmorning: