Adding an attribute to Shapefile

NEV

Using GM 22.1 I have a shapefile linerepresenting a proposed trench 87km long. The line is divided into 183 numbered segments of variable length. I wish to add an attribute to each segment representing sensitivity of landscape on a five-fold scale: Major, High, Moderate, Low, Minimal and to display the line with each segment coloured according to the sensitivity scale. (In the example, the line is random coloured according to segment number).  How do I do it?