The game is played by guessing a colour palette and the opponent mentions the number of Cows (for the colours which are present in the secret colour palette, but not in the guessed positions) and Bulls (for the colours which are present in the secret colour palette, and also in the exact positions).
For example, if the secret colour palette is red green blue yellow, and the first guessed colour palette is orange purple pink cyan would result in "0 Cows, 0 Bulls" (since all the guessed colours are not in the secret colour palette). If the guessed colour palette is orange purple pink red it would result in "1 Cows, 0 Bulls" (since red is present, but not in the exact position), and if the guessed colour palette is orange red blue yellow it would result in "1 Cows, 2 Bulls" (since blue and yellow are in the right positions, while red is in the wrong position). The game would continue until you guess the right number with "4 Bulls".