There are various reason why one wants to tone hair.
Toning on dark hair.
Let’s say someone wants to dye their hair but they don’t want to use a high percentage of peroxide. Now we usually use a 20 volume (6%)peroxide or a 30 volume (9%)peroxide for hair colours. These peroxide’s help to lift the colour you desire to achieve. Now if one doesn’t want to use a high peroxide one can use a 10 volume (3%) but the colour will not lift much it will just blend. Let’s say you have a pitch black and you want to go dark brown and you have no grey hair you can do this with a 1.9% peroxide. But if you have white /grey hair you have to do this with a 10 volume.
Toning bleached hair.
Having bleached hair can have its disadvantages. Our hair is made out of pigments, of course. So when bleaching to lighten the hair red pigments can appear easily.
For example let’s say you want to achieve a grey ash blonde but you have red patchy pigments. You can mix a grey toner with a green toner to even out the hair colour. If you bleach your hair and you have an orange pigment but you want a wild colour like blue ,pink purple and so on. You can just tone it with the desired colour.
But if you want a pastel colour you first have to get it ash blonde. And with an orange pigment would be hard but not impossible. Mix an ash toner with an blue toner. You may need to repeat this step every time you need to tone your hair until there is no orange left to tone down.
According to this colour chart you can hair according to your pigment. For example if you have a yellow pigment, opposite the yellow in this chart is purple therefore use a purple toner to tone down your pigment. This color shell shall be very helpful to tone hair according to your situation.