Some people prefer to use a bleach bath on the hair rather than normal full strength bleach to remove remnants of colour etc. Generally a bleach bath should be less harmful to the hair as its diluted bleach however caution should still be taken as it can still cause similar damage to bleach if in contact with the skin or left on the hair too long for example. As a general rule a bleach bath is diluted bleach: 1 part bleach powder + 1 part developer + 2 parts shampoo (note, some people may use 1 part shampoo and 1 part water instead of the 2 parts shampoo). As we are not hairdressers we cannot say if a bleach bath would be the right choice for you and if you are intending on doing any procedure on the hair involving bleach we suggest you research it well first or consult a hair professional for advice.
Some people also refer to Bleach Baths as Soapcaps and more information can be found here: http://www.hairfinder.com/hair3/soap-cap.htm
ADDED - Although many people refer to a Bleach Bath as being the same as a Soapcap sometimes they can be referring to a Soapcap meaning to wash the hair with a detergent product like Washing Up/Dishwashing Liquid instead so please make sure which version of 'soapcap' you are looking to do before attempting anything. With the dishwashing liquid type of soapcap this can fade the hair quicker in some cases but can also damage it in the process (albeit causing less damage than normal bleaching).
For more specific information on bleach baths or soapcaps please post a question in the main forum.
Is this a good way to get the remnents of a semi perm dye out to change colour then?
It can be if you dont want to bleach. As its a weaker bleach mix its not likely to make a huge change so dark brown hair is unlikely to go pale blonde for example, but its handy for removing remnants of a dye in order to dye a totally different colour (e.g. removing pink from hair so you can use a green dye).
Yeah i've found it pretty handy to get out the last remnants of a colour, but it can make hair feel pretty dry (as i generally mix it with clarifying shampoo too for that extra lift). So i only ever do one of these once a week and deep condition afterwards. I'd probably advise avoiding this on damaged/really bleached hair.
worked pretty well,moved the majority of my pink but left a little but the colour has been in and reapplied since june. it did manage to lift my roots really well and a lot less harsh that a straight bleach!
Soap caps are also often referred to as bleach baths. If you are wanting to know what a bleach bath is, this is the procedure.
sins_n_needles has made a great step by step guide to bleach baths which includes photos of what's needed as well as how to mix and apply it. It's a great guide and instead of moving it here I have stickied in the Bleaching & Lightening Hair section. You can see the full guide here: bleach bath tutorial with photos