feline fanatic

You need to first decide if you are going to do traditional attic windows with the Y seam or the easier method done with a HST in the corner.

To get the illusion of depth is usually done using two values in the same color family, like a black and a medium gray for example. Or a medium gray and a light gray. I have seen some done with stark changes in the two units (like two entirely different colors or black and white) and definitely prefer the same color family in light and dark values. It really doesn't matter which value you use for the "frame" and the "sill" just keep it the same throughout. Although I usually use the lighter value for the sill and the darker for the frame.

If going with the HST assembly, solids show a seam line more than a print. If going with the Y seam it doesn't matter.

Also you need to decide if you want to make the windows with or without sashing. That is preference. I find that the ones with sashing are more appealing to me when the sashing is very narrow. I prefer no sashing if it is a quilt with many "windows" but ones where you are taking a single large panel look nicer with a narrow sashing between a few windows.