If the surface temperature cools and the radius expands, chances are, the sun is running out of hydrogen fuel and will be switching to burn heavier elements and becoming a red giant. The sun would likely shift from yellow to orange or red, and even though the color would change, the luminosity would actually increase along with the size. Since the sun would be expanding to 7x10^9 mi (I assume miles, not meters?), then I would expect Earth to get pretty hot for a while. If Earth survives red-giant contact and is not simply engulfed, it could potentially move away from the sun, and its atmosphere might be pushed away as the sun grows closer and closer until, eventually, the atmosphere would be gone. If the atmosphere of the Earth is gone, then the planet will burn during the day and freeze at night, which means it would get both hotter and colder. Red giants can shrink and expand several times as they switch to burning different fuels.
Also, much of this is speculation since scientists think Earth might survive the expansion. I suggest checking out this video and the sources below: https://www.youtube.com/watch?v=0I33YxlQpvU
References:
https://science.nasa.gov/sun/facts/
https://science.nasa.gov/sun/
https://www.space.com/22471-red-giant-stars.html
https://www.e-education.psu.edu/astro801/content/l6_p2.html
