Please do not rely on the red protective conductor either. Depending on the location, it can be anything, e.g. a switched phase. This means that despite three conductors, a "classic neutralization" can suddenly occur anywhere, and a retrofitted residual current device (RCD) may be ineffective or cause continuous tripping.
Be careful also with "three-phase lines": if at that time a stove or instantaneous water heater was connected, then red is one of the three phases. That is then blue, white, red, gray and has no earth. And generally, for that age: so many additions have been made in the meantime without documentation that everything must be questioned. No color has to comply with the standard; everything must be measured. I found switched phases on green-yellow in my installation.
All colors, meanings not standard-compliant:
(including series connection of two lamps)