As noted by others, these coins are solid cupronickel, not plated. More modern versions of the coin are plated steel, but these original 1975 versions are solid alloy, with no plating.
When a solid cupronickel coin is "coppery colour", the reason is almost always "environmental damage" - specifically, from burial in soil. Humic acids leach out of the soil and attach to the metal, giving it a soil-brown colour, which can look very similar to oxidized copper, as in the examples in NumisRob's picture. The exact colour it turns will depend on the chemical nature of the soil in question.
The "chemical stripping" thing isn't likely to happen, but might be possible: copper is a minority component of the alloy, but nickel is more chemically reactive than copper, so you'd need to find a very particular stripping agent that reacted with nickel but left copper alone, then leave the coin in that solution for long enough. I'm not sure the resultant coin would look as clean as this; I'd expect some pitting and roughness.
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