Hello kipayne and welcome to the Community.
Is this window in a location that will get wet, like in the shower area? That will definitely effect your options.
There are several ways you can change the appearance of your window sill. Home Depot carries Rustoleum Countertop paint kits in a variety of color options. You can apply it with or without the decorative accent flakes.
If you have the space or clearance you could install tile directly over the marble sill. Tile will give you a great deal of color size and style options.
Or again if you have the space or clearance you could install a thin layer of wood or laminate flooring.
Depending on the shape of your marble sill you could possibly use a product called Cap-a-tread. Cap-a tread is a laminate stair cover that has a variety of bullnose sizes. Some have bullnoses that cover a 1” thick stair tread.
Other models have a bullnose that will cover stairs treads that range from 1-1/8” to 1-3/4” thick stair treads.
Check out the install videos by clicking on the box with the arrow under the products picture. And for more product information, installation guides and details on the sizes, select the specification tab.
You would have to remove the window and do some demo work to remove and replace the sill. Removing the sill is a major project and then you still have to install something in its place.
I would love to see before and after pictures.
Thanks for your inquiry.
Hi kjpayne, and Welcome to the Community!
I work in the Doors & Windows Dept at the Home Depot, and I have a different opinion on your question. Usually, in most cases, the window stool is something that you can replace and does not go past the lower sash, as you see in the picture below indicated in red--the window stool is installed after the window in installed and the stool is part of the finished carpentry work. I believe your contractor might have been a bit leery about removing the marble stool.
If you have pictures showing the stool protruding to the outside, please post them here. I know if customer that have marble stool, and when they replaced their windows, the installer kept them intact, and re-installed them, but they do break easily.