Phenol red-free formulations are manufactured using DMEM that does not contain phenol red, so as a result the product is colorless.  Phenol red-free formulations are recommended for assays that require color detection.  Phenol red may exhibit estrogenic effects, so we recommend using phenol red-free Matrigel matrix if estrogenic effects are an application concern.
Phenol red containing formulations are manufactured with DMEM containing phenol red.  The color variations that are observed in frozen & thawed Matrigel matrix products that contain phenol red may range from straw yellow to dark red. This is due to the interaction of carbon dioxide with the bicarbonate buffer and the phenol red.  The color variation does not affect product efficacy, and will disappear upon equilibration with 5% CO2.