DMEM contains phenol red, which is solely a pH indicator. At physiological pH, the media is a pink-red color. When the media is acidic it turns a yellow-orange color as you describe, and when basic it turns a deep purple (such as if you add bleach).

Why does cell media turn yellow?

During the cell growth, the medium changes color as pH is changed due to the metabolites released by the cells. At low pH levels, phenol red turns the medium yellow, while at higher pH levels it turns the medium purple.

What color should Dmem be?

The color of DMEM at its proper physiologic pH of 7.4 is orange-red; the color changes at pH 7.8 to purple, at pH 7.6 to blue-red, at pH 7.0 to orange, and at pH 6.5 to yellow. Bacterial contamination of a culture is the most common reason that medium becomes overly acidic.