What does the SDI filter paper color mean?

Filter paper coloration after the SDI test:

1. Slightly yellow – The possible problem could be the presence of iron and/or organics in feed water. Check the source.
2. Reddish-brown – Positive indication of the presence of iron in feed water. Take corrective steps for the removal of iron.
3. Gray – Indicates the presence of carbon fines. Check the carbon filter, remove carbon fines.
4. Particles present on the filter paper – Check the cartridge filter and the filter elements.
5. Dark Black – If the color dissolves in the acid, it indicates the presence of manganese in the water.