Copper Sulfate is used as a mordant in natural dyeing processes. 
Copper sulfate will improve light fastness and was fastness for all natural dyes and changes the colour. It brings out the blue and green hues of the natural dye. Use with logwood for deep night blues, intensifies all yellow dyes to a more mustard hue. 
 
For use; weigh 4-5% of copper sulfate per 100 grams of dry goods, dissolve in hot water and top off with cold water to cover the fibers. As low as 2% will work, try and see the difference for yourself.
Let the fibers sit in this mixture overnight, stir in between. Or heat for 1-2 hours to below a simmer.
 
About toxicity of copper sulphate read here.