The light portions of the columns are new pieces of marble. Over time they will weather to match the older pieces. The new stones must be cut to fit exactly as no two columns are alike. If you look closely you can see that the columns are built of "drums" of marble. Each drum has a square hole in the center, through which is inserted a metal rod to hold them together. The original builders enclosed the rods in lead. Restorers in the early 20th century used iron rods to replace them; the iron rusted and cracked the marble, leading to extensive damage. New rods are titanium.
I think that the wall you can see behind the columns may be part of the inner room, or cella.