For many natural dyeing processes, checking the pH is critical.  For example when using indigo, it is always beneficial to check the pH for optimal dyeing performance. such that it is around pH 10-11 at the time of setting up the vat.  Checking the pH is also important for when tending a vat as it determines whether more alkaline base should be added (e.g. soda ash).  Equally it detects, if the pH conditions are too low for the indigo dye molecues to be dissolved and released (an alkaline pH is necessary for this to work).
 
Likewise when using procion MX dyes for cotton, or acid milling dyes for wool and silk, testing the pH is essential to make sure the dyes work most effectively. For Procion MX, the conditions need to alkaline so soda ash is used to raise the pH, and for acid milling dyes acidic conditions are required, so the pH is adjusted with citric acid (or vinegar - acetic acid) to get the conditions right to fix the dye properly in the dye bath.
Here we offer a handy pack of paper pH testing (litmus) paper which includes the full pH colour chart.  The pH strips can be torn off and used to do the pH check.  In each pack there is 80 tear off strips.
 
The scale of these pH strips runs from 1-14.  The interpretation of the pH scale is as follows:
 
pH 1-3 - is a highly acidic substance (required for acid milling dyes, and sometimes to liberate natural dye stuffs.
pH 4-6 - this indicates an acidic substance.
pH 7 - this is a neutral pH as is best for when washing wool and silk, and also neutralising either acid or base (alkaline pH), and after dyeing with indigo to neutralise the alkalinity and help fix the dye into the fabric or fibre.
pH 8 -11 - this is an alkaline pH.  This pH is best for when using procion MX dyes when dyeing cellulosic materials such as cotton, rayon and bamboo.  It is also the optimal pH for when using an indigo dye vat, where an alkaline pH is required to fully liberate indigotin.  Always take care at this pH range when dyeing wool, as alkaline conditions have the tendency to denature protein based fibres like wool, so it is recommended to keep closer to pH 9-10 when dyeing wool with indigo.
pH 12-15 - this range is highly alkaline, and is usually best avoided - only concentrated alkaline scouring washes for cotton or other soda-ash based products should reach this pH level.  
 
Always wear gloves and protect the skin at both extremes of the pH range.