Sanding and revarnishing areas of different shades (wood floors)
If this is just a clear coat on the wood, then you won't be able to "fix" it. If the coating was a stain you could sand the light area and re-stain with a slight blending overlap into the dark area. Then sand the blending until the color looks blended. Another way you might be able to correct this, if it a clear coat, is to get a slightly gray stain, or light color to match the dark area, then sand and blend and sand then coat. But as was stated above, this is a sunlight issue, and there is no way to stop this from happening unless you do the entire floor, then cover it with a UV stable coating.