First of all, who is paying for this house?? Why aren't your wishes being followed. If it's done, paint them..... https://www.bobvila.com/posts/213975-paint-marble-window-sill?page=1

You might look into a product called Timbersil for your windows. It's the same southern yellow pine that they start with for pressure treated lumber. Instead of the nasty chemicals however they inject it with sodium silicate and then bake it. the silicate fuses into a glass like material in the pores of the wood making it inert; rot proof, insect proof and won't burn. Best part is it looks like untreated wood and works about the same. I just completed a big pergola project with it and really enjoyed working with it. It's kinda particular in how it takes paint (you've got to sand the wood before you paint) but it seems to take paint well. You could also look into using mahogany but without being sure of it's origin, I'm reluctant to use something that might be contributing to rainforest destruction.

No-casings and painted sheetrock returns are the right look for a modern house, and the major brands of windows have all gotten on board with all-fiberglass or for Andersen, all fibrex, windows, this class of windows having relatively shallow frame depth, and all offer dark colors same outside as in, to achieve this popular new look in "modern." The #1 favorite color is black, followed by dark bronze. To do it right, the carpenter needs to properly shim the opening before the rockers get there, so reveals are consistent all around. Insider Carpentry channel on YouTube has a great vid on how to do this right. The rockers need to mask well, then use a tearaway bead at the window side, and a good corner bead at the outboard sides. The best look, IMHO, is to have a 3/4" thick sill at bottom with the same window reveal as the sides, so the bottom of the opening is not shimmed in as much as the sides. To get this right, the window sits up on shims at installation time and the bottom of the opening is only shimmed to get it dead level and flat for the drywall corner bead. That sill gets no apron under and has to have a good plate-like bed. Anything more than 3/4" looks to beefy. Sills should have tiny returns. Here is a 3D render done in Chief Architect of a window with sill and another without. The windows are modeled at the same dimesions as the all-fibrex Andersen Series 100, the wall is 2x6 framing, and the depth of the returns to the drywall face is 4-9/16". Note the very shallow bullnose on the sill edge, and the small return. A painted wood or stone, metal, or composite sill is going to be more durable and easier to wipe clean than painted drywall, no matter the paint. And as for the dust on those vertical returns at jambs, or horizontal across the head, and durability, you have all kinds of drywall corners eleswhere in the house. Why are corners at windows going to behave differently. If your builder has not done these no-casings openings before, think twice before asking him to do it. If he has, insist on seeing examples of work. You don't need to be a first time event for the builder, his carpenter, or his sheetrocker.

We’re painting our house alabaster and windows are black. What color do we paint the window sills? Black? We’ll also have black gutters. The paint shown in picture is primer. Thank you!