Various substances can be added to the dyebath to increase the range of colours, or shades of colour obtained from one dye. These are generally known as 'modifiers'. Some dyes are pH sensitive so can be changed by the addition of an acid or an alkali and 'iron water' darkens or 'saddens' a colour.
Iron water is easily made by putting rusty nails into a jar and pouring on one-third distilled vinegar and two thirds water. Leave this for a couple of weeks and you get a good brown sludgy liquid. It can be topped up as it is used.
But where to find rusty nails?
He collects these on his magnetic collar and deposits them onto his cat flap - this is at some personal cost, as once they are there the flap sticks and he can't get in or out!