Which wire is positive? Answered

If it's an electret mic, the wire connected to the case (measure with a multimeter if not obvious) is ground.

For EU mains supplies
BROWN is HOT for one phase, Black for an other, and GRAY the other, BLUE is neutral, green/yellow is earth.....

Which for OLD UK phasing (Blue-red-yellow - and black) makes for interesting fireworks.

Black is usually neg. but it may not be conventional color coding.  Unless it's an important/expensive piece I'd just choose a color and hook it up if it doesn't work then reverse them.