Officially colouring or toning is considered "damage" but as other have said, this is very subjective. Some might offer you a premium for them, while others won't touch them. It's all eye appeal and personal tastes. Also, I've heard that toned coins can/will change over time. It has to do with the environments past, present and future. While many have tried to manipulate the process, few have seceded, and as stated the colour you see presently is no guarantee of where the tones/colours may go.
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