I have noticed a great deal of variability with the colour of British copper coins. I haven't noticed any pattern.
It's not something happening at the mint that's causing it. The official composition of British bronze coins (95% copper, 4% tin, 1% zinc) remained constant from 1860 to 1922. At this time period (beginning in 1897), farthings were being artificially blackened in the mint, so they would not be confused with gold half-sovereigns. However, this would not have happened with pennies.
I think it's simply a case of different life-histories of each coin. Sulfur in the atmosphere and environment will turn copper coins black. Coins circulating in industrial cities will turn black quicker than coins circulating in rural areas far from the pollution.
Don't say "infinitely" when you mean "very"; otherwise, you'll have no word left when you want to talk about something really infinite. - C. S. Lewis