Purpose: "Priority" field allows to specify relative priority of the record (is it "Urgent", "High", "Low" or any other priority you create).
Scope: Field is global.
Custom color may be associated with "Priority":
Colors can be specified by using a color name, or by using numbers to denote an RGB color value. For example, the color value #FF0000 is rendered red because the red number is set to its highest value, FF (or 255 in decimal form).