Specifies a method for determining the resistance of the colour of textiles of all kinds and in all forms, except loose fibres, to exposure to atmospheric oxides of nitrogen as derived from the combustion of chemically pure butane gas. The method may be used for rating the colour fastness of dyes by applying the dye to textiles by a specified procedure and at a specified depth of colour and testing the dyed textiles. The change in colour is assessed with the grey scale.