[updraught]

My guess is the halogens are sucking too much power for the wiring. Or trying to ...

Try connecting one light direct to the battery with heavy cables and see if it is then white. If it is, you need a relay and thick cables from the lights direct to the battery, thereby bypassing the light switch.

Do a search for "headlight wiring upgrade" for some ideas.

PS. This is a problem on some modern vehicles also where the manufacturers are too cheap to use anything more than two strand copper wire.