Can this be mixed into regular gray concrete to achieve a lighter gray color? If so what should be the formula or ratio?

White portland cement should not be added to pre-blended concrete mixes, which are already optimized to meet specifications with minimal shrinkage. The extra cement will cause shrinkage. Instead, the white portland can be blended with gray portland cement to make a lighter colored mortar. The recommended blending ratio is one part (by weight) of cement to two parts coarse sand to three parts gravel; ie. One 94#bag of cement to ~200 pounds of concrete sand to ~300 pounds of gravel.