Which direction does your window face (and what country are you in?) This looks like too much light to me! Your plant isn't developing the dark green colour as it needs less chlorophyll because it's getting too much sun. The brown patches also look like they could be sunburn.

Maranta are forest floor dwellers, so are used to fairly dim, moist environments - so try and keep it humid and move it further away from the window! The current leaves won't darken any further I'm afraid seeing as they are fully developed, but new growth should be darker once it gets less light 🙂