The colors are intended to make the load and line side wires easier to distinguish. Black stands for normal phase, brown usually means a switched phase.
Covers don't help much in this case. The new RCD is exactly the same width but leaves a big gap at the top and bottom. Looks like cardboard and scotch tape, maybe some kind of sheet plastic instead of the cardboard.
The house was built in 1913 and parts of the original wires are still in use. In our case there are 2 ungrounded receptacles that were rewired in the 60ies or 70ies, meaning the wiring is still perfectly safe, only without a ground. In one case the conduit has 2 90 degree bends so I can't get a ground through, the other one is wired with 2w Romex buried in plaster. I don't really care for rewiring them since they only serve stuff like table lamps, TV, VCR,... and there are plenty of Schuko receptacles around if I need to plug in something grounded. The pro version would be to run a 4mm2 ground wire along the baseboards and up to the receptacles, but I neither like working with 4 mm2 solid nor having yellow/green wires running up my walls. Besides I'm a bit nostalgic.
What gets me a bit about the RCD requirement is that I have to dump the old 100mA RCD that perfectly fits the panel. And it still works perfectly, even though it's 25 years old. We test it frequently, and it always tripped on occasional ground faults when plugging in faulty equipment.
In Germany only bathroom receptacles have to be RCD-protected.
The last electrician we had just hooked up the meter, even though the wiring was horrible, and there were 2 Schuko receptacles in the entire appartment. Anyway, that guy retired since, so we can't call him any more.
