Is this correct? Positive tests in the fermentation tests turned the test media from red to yellow. Why does this color change occur? The acidic by-products of fermentation are changing the pH , as shown by a color change in the pH indicator in the media The bacteria are catabolizing prodigiosin in the media Bacteria are yellow in color, and they turn the broth yellow as their population grows while they ferment the carbohydrates in the media The by-products of carbohydrate fermentation are yellow in color.