I use either Osmo, Joico or Pureology shampoos and conditioners. You can get them from the hairdressers.
Also, look for conditioners containing Argon oil or pure coconut oil.
Never ever use washing up liquid on your hair, not unless you want to look like Worzel Gummidge. If you've had your hair dyed chemically using a peroxide based product, the colour has penetrated into the hair shaft.
Using fairy liquid or any other harsh detergent based product on your hair will permanently damage the cuticle scales and this allows some of the artificial hair colour molecules to wash out. So it might look a bit lighter... However, once the scales are damaged, no amount of conditioner will repair them. You need the cuticle scales to lie flat so that light bounces off them. That's why hair in good condition (or straightened) looks so shiny.
If the new colour is too dark, get it fixed properly.