A dog or cat food kibble can vary in size and colour from batch to batch. Colour variations will occur especially when there is no natural or artificial colour added to the formulation to keep it consistent. The reasons for this are mainly because of the raw materials that are added that can vary in colour. Raw materials such as animal protein meals especially are different shades of brown depending on what process they went through, where the animals came from and what diet they were fed, and what parts of the animal were used ie. what the formulation was when rendering the meal.
Size variation is usually the result of the manufacturing process, how big the extruder is and how much pressure the kibble is put under when forming ie. density. Specific ingredients have different effects on how the kibble is formed and therefore the machine operator plays a major role in adjusting parameters to get the size and shape correct. When running a large production run, it is somewhat easier to refine all the parameters to result in a consistent size kibble. On smaller production runs (which are normally done on smaller machines), there is not as much control over these parameters and slight inconsistencies may occur.
See below some links to scientific articles relating to above:
https://books.google.co.za/bookshl=en&lr=&id=e1OkAgAAQBAJ&oi=fnd&pg=PA5&dq=raw+materials+colour+variation+in+dog+food&ots=uiVzNU_Ope&sig=6encAT7N9dxe08PsXrOtnWxTkZE&redir_esc=y#v=onepage&q&f=false
https://animres.edpsciences.org/articles/animres/abs/2001/03/priolo/priolo.html
https://www.sciencedirect.com/science/article/abs/pii/030881468290070X
https://www.sciencedirect.com/science/article/abs/pii/S0377840116303625
