I am trying to differentiate whether the plumage is gray or black with certain values. I have bird's image (jpeg) with a scale (ruler) and color card beside the bird's body
A good approach is to reference the sample to a colour atlas in which ordered arrays of colour examples are available. Think if you will of paint choosing cards. Royal Society Publishing have a comprehensive pdf file guide to bird plumage colour and commercially, atlases such as Pantone and Munsell are also available.