I would swap them back myself.

AFAIK on a mini the yellow/white wire goes to the + (positive feed from
ignition barrel) and the black/white goes to the distributor/points AND goes
to the clocks as a sender wire for the rev counter.  The + is the power
source for the coil.  It is turned on and off by the points in the
distributor.  When the points turn 'off' the coil the magnetic field in the
primary coil collapses very fast.  This creates a very high voltage in the
secondary coil which is shorted out to earth via the spark plug.

The electronic ignitions are slighly different only in that they dont
actually short out 12v, they use a hall sender.  I remember fitting a late
metro elecy distributer to a mini engine to get the elecy ignition.  It was
really easy and only involved fitting the two connectors from the metro over
the connections on the coil - metro wires were same colour so y/w to
positive and b/w to negative.  All that had to be done then was to fit the
yellow and white lead from the mini loom to the positive side of the coil
too.

I think the later electronic ignition cars use 9v coils and ballast
resistors as they start better with these.  Not too sure with lumenition
systems but I've been told they're very reliable once set up.

hth

Chris