C, will remain liquid until temperature cools to 40-42 deg. Very few microbes can degrade agar, so it is normally not a source of C, and acts as inert gelling medium. Useful for photoautotrophs, also in some experimental situations where want to select mutants unable to use certain compounds, or for radioisotope labeling. You would prepare synthetic medium with sources of C, P, and S, but no N source. Organisms would be unable to grow unless they can fix nitrogen from air.
MacConkey agar has color indicator that distinguishes presence of acid.
Bacteria that cannot ferment the same sugar will grow but not affect pH, so colonies remain white. Note that it is possible to design a medium that is both selective and differential.