I worked on military avionics in a previous life and all of the planes I worked on had miles of white wires. The reasons given to us were:

1. Cost savings when buying.

2. There are hundreds to thousands of wires in an aircraft, but there are NOT hundreds to thousands of different colors of wires (even if we allow for stripes, etc) that are readily distinguishable to an overworked airman hunched over in a dim, cramped avionics bay trying to fix a plane that needs to take off for a mission in an hour.

If you are lucky, the wires are numbered. But even if they are, you typically identify a wire by its connector pin and PRAY that the fault isn't a break somewhere back in the wiring harness.

Tangentially, this reminds me of stories from my dad who got some kind of special award for having made their ship radar the best in the fleet.

Sometime before that, he got a lot of flak for having neglected one of the standing rules, to label everything as you take it apart and put it back "the way you found it". He decided to break it down and put it back the way the technical documentation said it should actually go. This seems to be part of the reason his radar performed better than the others after teardown maintenance.

Surely you mean F-15 right? (F-14 was exclusive to the Navy + Iran exports)

Cheaper to buy huge spools of gray clad wiring than a lot of different color coded wires? Also you don't have to stock a lot of different colors for repairs.