Light reduces the iron center of the ferrioxalate ion, and oxidizes the oxalate ion into carbon dioxide. Since the green color of the solution is due to ferrioxalate ions, a loss of them will make the solution browner, which is the color of iron.

If there is excess oxalic acid present, then the extra oxalate ions can form ferrioxalate ions in the absence of light, causing the solution to revert to green.