Hope i'm not telling me granny how to suck eggs here........ but Gardiners fibredyne filters change colour when you need to change them ?
If you are just using the Fiberdyne as a single filter (no sediment filter) then the Fiberdyne will be trapping sediment particles bigger than 5 micron. The change of colour is the sediment from the water.
I have to use a sediment filter as our water is terrible sometimes. The sediment filter is red with sediment but the Fiberdyne is clean right to the end before replacement time.
The fiberdyne filter is also 5 micron, so particles smaller than 5 micron will pass through the sediment and the Fiberdyne filter.
.