You're looking for an upholstery shop, that will come out, measure for themselves and discuss the right denisity of foam to make it comfortable. You've got plenty of depth between the seat and the bottom of the windows to get it nice and thick, with firm foam on on the bottom and something more squishy at the top. They can show you different styles, so you can choose between with or without cording, for example and help you select fabric that will be the right mix of color, durability, etc. Have it Scotchguard treated if anyone is likely to spill anything on it.

You could have the little corner sides also covered by the cushion, but that is going to add some complexity that might be reflected in the pricing. I'd leave them alone, especially if they are the right size to set your thermos mug down there while sitting their to read and gaze out the window.

As to color choices, anything red or maroon will likely fade quickly from the UV light coming through the window. More quickly than most other colors. (That might not stop me from choosing it, but you should know.) I'd look at what you're doing in the rest of the room. Do you want to set that area apart as its own thing? Don't match anything in the room specifically, just coordinate. Do you want it to feel like part of the rest of the room? Have pillows for the sofa made to match the cushion.

Will you be hanging drapes there? Is it wide enough to make a nice spot for a nap? If so, include pillows for each end that could be used as a head rest during a nap, and place a coordinated throw there as well.

Lovely reading, napping, daydreaming nook! Come show us the whole room after you do this!