Let’s propose you want a dark background for a rug.
Look for some wool that is already on the way to being darker. I like to use a four value range on the value scale for this type of work ie: #4 #5 #6 #7.
It helps to save on dye if you are starting out with medium values for dyeing darks.
Here are a group of close values, use these to judge your selection.
Gather up different wool that fits a “mostly”, they might be mostly green, mostly grey, mostly brown... just choose the mostly that suits your purpose or what you have on hand. Use textures and little bits you might have leftover too. No need to worry about running out. Just make sure they fit your chosen “mostly”
Here are the Mostly Greys for Old Stormy before dyeing
Wet your wool as usual with synthrapol or the wool wetter of your choice, shampoo without conditioner will work if you have nothing else.
Chose one dye from your selection that will get you the results you desire. Keep in mind what happens to certain colours when they are dyed with another, for instance, blue dyed over brown becomes tealish. Maybe you will want to add some black dye to deaden or some brown dye to dull according to what you need your dark areas to look like. Make that judgment as you dye. It takes about 1/2 tsp of darker dyes to make this much wool darker and keep in mind lighter dyes such as yellow will not make a dark colour for you.
A series of blue greenish wool over-dyed with black will give you a cool black. For a warmer one look for a “mostly” that are brown.
Gather 6 pieces of random grey textures and solids equaling 8 oz or 3/4 of a yd. Arrange them from light to dark and add them in that order. This will let more dye go into the lighter wool and less for the darker wool keeping the marriage close in values.
These were over-dyed with Majic Carpet Dyes. 1/4 tsp. Blue + 1/8th tsp. Black in a dye bath with plenty of room, with minimum stirring. Add 1/4 tsp of citric acid after 10 minutes.
When water clears rinse wool well and dry.
When you hook these types of backgrounds try random irregular sections of colour dispersed without planning.