New iphone 6s plus invert colors shows white text on cloudy gray background-where is the black? (not the black and white function)

Low vision user issue: invert color on my iphone4s shows white text on black background, but my new iphone 6s plus shows white text on light gray background. The light gray background in NOT low vision friendly. Is there (or where) is a workaround, or setting that will darken the light gray background? (Would prefer black or charcoal gray.) Anyone have a working suggestion or fix?