Toning

I will admit that toned coins do sell better than none toned coins. They have more eye appeal and are sought after by most collectors. Toning is the patina that occurs on uncirculated coins over a period of time and the brighter the colours such as blue, red, purple the more desireable they are.
As for obtaining those colours everyone has there own method as for myself , I have had some success this way.
First you need a coin that is high in silver content such as 925 and preferably as close to AU as possible, then place it in a brown envelope (the smaller the better) and place it in a dark place such as the bottom of a drawer and check it once in a while until the dsired stage has been reached.
I am currently doing this with a USA Mercury dime and it is toning quite nicely..

Some would consider "toning" to be another word for "tarnish".

Its the chemical interaction between the surface of a silver or copper coin and the air, surfaces or containers.

If the air is smoky, the coin is probably going to turn yellow.

If the coin is against certain wood surfaces, it will turn black.

If the coin is baked inside a potato in the oven, it may have a rainbow finish (or be completely destroyed).

don't try to artificially tone your coin ! It takes time and some element of storage to do a stunning job and if I can come across a coin you toned on your own I will tell and others collectors too .

It's possible in the future, toned coins will lose some of their popularity and value if collectors keep trying to perfect fake toning. Now every time I see a toned coin I study if it has a fake "look" to it.