From time to time I have to do some electrical maintenance, e.g., connecting new lights or integrating flush-mounted switch. The by far largest problem here is to identify the right wires. Even though I know the current color specification, I rarely see them (at least in my recent flats).
This page aggregates some information to help me identify the right wires.
Wikipedia has the following table:
The old convention can be usually found in houses build in the 1960s and 1970s.