i understand that for this series of pictures, you want to use natural light
don't be afraid to use flash :) just diffuse it, so it will blend with the natural light. i'm guessing that you're using a pop-up flash. why don't you try tying a coffee filter around it and see how warm the colors come out instead of direct flash.
Also, you could put a white card under your flash, slanted at a 45-degree angle pointed towards the wall for a bounce fill.
Try visiting this website,
http://digital-photography-school.com for a lot of great photography tutorials :)