After you have cut your hair to the desired length, you can immediately dye your hair anytime after that (presuming your hair has not been previously dyed before). Be sure that the hair is dry first, before colouring.
If the hair has been previously coloured and you just want to dye your roots soft black or cover grays, I suggest you just "touch up" the roots and leave the rest of the coloured hair.
You can preview the outcome colour of your hair by doing a strand test before you dye. Cut off a small snippet of hair, tape it together at one end to hold it, then mix a bit of the colouring and apply to the snippet of cut-off hair. Time how long it takes to dye that strand and this timing is how long you should dye your hair for.
