Whether you use a boxed hair colour at home or you get it done professionally at a salon, the recommended wait time before washing your hair is 24 hours. If you use a double-process colour (such as a base colour and highlights), try to go 48 hours without washing your hair to allow your hair and scalp to build up their natural oils again.
If you have unexpected results such as an overly vibrant, dark or ashy colour, the professionals at Schwarzkopf (a leader in the hair colour industry) suggest washing your hair immediately.
Wait a minimum of 24 hours after any colour process for best results, unless you want to tone down the colour-in, in which case wash it immediately. Always follow up with a deep conditioner to restore moisture to the hair.
The first day after colouring is your best chance to get rid of it, if you change your mind about the colour.