When the RPMI is yellow...

"Notably, genes involved in cholesterol biosynthesis and related processes were largely up-regulated or suppressed in response to low (1%) versus normal (5%) or high (10%) CO2 exposure, respectively. This effect was largely independent of pH changes known to occur with varying CO2concentrations"

https://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.3002401