[MMI Enterprises]

..the cement part of concrete can come gray or it can come white or you can add any other integral coloring to the mix or broadcast color it with powder when laid.
You can oxalic an old slab to some degree or you can apply an acrylic white stain. That would require real finesse in prep and application to get it to last without top coating with industrial type permenant coatings. One or two real lite mist coats of quartz white will likely last longer than say a heavy dose as it will minimize traffic wear and chipping.