I really cannot see how any fluctuation in copper prices/ratios would really effect Rolex decision to cut down on the copper content. Bearing in mind Gold and copper move pretty much in the same corolation even if there were a shift the price difference ratio it would be minimal. Sorry to be a pedant but you asked the question so here goes...
The maths at today's heady prices:
spot copper @ $3.60/lb
/16 = 22.5cents/oz
spot gold @ $1,730/oz = Gold to copper ratio of 7688:1
That is to say an 1803 total gold content value (on a bracelet) right now today is USD$4,166 if the other 25% were pure copper (which it isn't) the total copper content would be worth ... $0.18cents!!
And that 18 cents is gonna be about double than the peak 1975 price (inflation adjusted - the actual would have been much lower at the time)
I think the colour difference in these watches is not due to copper shortages. Watches are just to tiny compared to a house's plumbing or wiring to make a difference to a serious manufacturer.
Either Rolex did it on purpose and there are 3 definite colours or they were just sloppy and colour consistency wasn't important to them.