Why is phenol red included in the urea medium while bromothymol blue is included in simmon's?

Phenol red is used as the indicator for the Urea Medium because the indicator detects the formation of ammonia caused by the hydrolysis of urea done by the bacteria. Bromothymol blue is used in Simmons Citrate Agar because it effectively indicates the formation of carbonates formed by the consumption of citrate.