Colouring is a very difficult and specialised job and a test strand should always be done before a full colouring is attempted to avoid any major colour disasters.
When hair is bleached it removes the hairs natural pigment. This is done in varying degrees depending on the type of hair initially. Also the product used will affect the degree of colour pigment removed. The strength of peroxide, plus the type of product used will also determine how blonde it will lift.
Very dark strong hair will first go orange and will require re-bleaching. The next degree will be golden blonde. Then the next bleaching will lift it to pale blonde. The more times you bleach your hair the more damage is caused to the hair structure.
Because bleached hair becomes much more porous, if you go into a swimming pool your hair can absorb the chlorine and turn it green.
Some scientific research also states that the presence of copper sulphate in water can turn blonde hair green. This will also affect black, all shades of brown and red heads but because the blonde has no natural pigment left in it, it shows.
Also if you have bleached hair and decide to darken it, if all the natural hair pigment has been removed to a pale blonde shade, the hair can become a green shade especially if an ash tone is used, because all the natural red tones have been removed. Your colour choice should contain some red tones to reduce the possibility of your hair turning green. Powder bleach is the strongest cosmetic bleach but can spoil the natural lustre of the hair. If this is used with 40% peroxide it will give the highest lift but should not be applied to the scalp as it could burn it. This should only be used with a streaking cap to protect the scalp for highlights. It can also be detrimental to the hair.
Some hairdressers will even use 60% peroxide for speed to achieve the lift quicker, but this can cause the hair to burn or become very brittle and straw like and can be dangerous not only for the hair and scalp but it can also cause blindness from the vapours.
Hair that is used for extensions can be Indian or East European Hair. East European hair is very strong and it takes a lot of bleaching to lift the colour. In many of the factories it is industrial chemicals that are used to lift the colour and then they dye it to the required shade. Therefore always remember that this hair has gone through many chemical processes before you receive it and this should always be taken into consideration before applying colour.
Remember to always do a strand test beforehand to check the result will be what you want.