Any litterbox may have a contrasting streak or mark of any other color, and the main color may also have variable shades and hues.
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.
Colors priced at $19.95 have a blemish. The violet and blue have a dark spot roughly a quarter inch in diameter in the center of the bottom.