With little of no oxalic acid it will be a golden amber color. With moderate mounts of oxalic acid it will be a yellow, with large amounts it will turn fluorescent green. The following are procedures for testing and making ferric oxalate. If a white precipitate is formed, this is silver oxalate, and indicates that there is free oxalic acid present. Note that the ferric oxalate solution should be at room temperature when the test is made.
Ferrous oxalate is the result of the decay by light or age of ferric oxalate.
This causes the precipitation of the platinum or palladium metal that forms the image in a platinum print. Your ferric oxalate should be relatively free of ferrous for good printmaking. There is a traditional test for the presence of ferrous oxalate. This involves testing with potassium ferricyanide. This test is unreliable in that it gives false positives when EDTA has been added.