Wonky Weaver™ Iron Sulphate (otherwise known as Ferrous Sulphate) is a Dye Assist and is usually used to change the colour of a natural dye.  It also makes natural dyes more light and colour fast.  It is more commonly used with cellulose fibres than protein as it can make protein fibres brittle on use.  The use of iron changes colours to deeper, darker shades.  
 
Recommended use is between 2 and 4% Weight of Fibre.