A leak usually shows up as a drop in ride height fairly quickly. Best way to judge height is by looking at the car on a level surface.
You probably realise that a leak in a front displacer hose will fill up the hollow top where the hose is attached, so the leak won't appear on the outside or under the car for some time.
I don't think the colour of the fluid is much of an issue. Most of it will still be green, just with some rust colour added. 7 years isn't long and the factory never set a refill period, unlike the coolant or brake fluids.
You could try inserting a small screwdriver under the dust boot lower edge to see if there is any fluid building up from a leak in the flexible part of the displacer.
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Hydro unit.jpg
And try to avoid removing the dust boot from its large end. Very frustrating thing to pop back on!
