Any litterbox may have a contrasting streak or mark of any other color, and the main color may also have variable shades and hues.
These marks range from none to barely perceptible to wild. The litterboxes with no marks or the faintest marks are shipped first and then progress to the most visible marks.
On a related note, the colors seen on your screen are almost certainly not the exact color they actually are. Allowances should be made for the variables inherent in creating and displaying images.