Brand new rear brake pad linings measures 6mm,
the lining & metal backing is 9mm.
The lowest I would let it go would be 4.5mm total thickness, that would give you a lining thickness of around 1mm to 1.5mm.
If you measure from the brake pad metal to the disc you can work out the pad thickness. The inner pad will be 1mm less in thickness then the outer pad.
By the looks of your brake fluid I would be changing it. There is no procedure that I know of on the forum, the steps are in the W/Shop manual.
I am busy making the Service instructions that will include these items for the mod book as we speak.