Patrick wrote:

I have some vinatge headphones (Sennheiser HD424) and want to attach a new
plug.

Which color wires are the positive ones?

The colors in one of the leads are red & blue and in the other lead black
&
yellow. (No wire is used as screening - there's just two wires in each
lead.)

My first guess would be blue+ red- and black+ yellow-, but here's how to
find out. First hook them up, arbitrarily starting with what I said, or
the other way - it's a coin toss. Listen to something in mono. You'll
have to find your own mono source. (finding a mono source is left as an
exercise for the student.)

If the resulting sound comes from the middle of your head, you got
it right. If it sounds like two sources on either side of your head,
you've got the polarity wrong on one side or the other.

I'm getting this from remembering (I think) that in the old 4-wire
phone lines, green was tip and red was ring, and black was tip and yellow
was ring.
--- http://en.wikipedia.org/wiki/Tip_and_ring

Hope This Helps!
Rich