<p>When you cut or break a PLA model (for example to remove support) it often leaves an ugly while mark where the removed piece was connected.</p>

<p>Sanding also tend to leave dull white scratches on the sanded surface.</p>

<p>What can I do to restore the white areas to the original filament color?</p>

A quick blast from a heat gun will very slightly reflow the surface texture and eliminate white marks.
However, it's important to avoid over-heating the perimeter layers or you'll see them soften and sag into the infill. So wait for the heat gun to get fully hot and then use a short duration of high heat. Let the part cool between attempts if you don't get it all the first time, or need to clean up a large area.
Incidentally, the heat gun will also help clean up strings from travel moves.