As Wolfman said - Walnut cleans better (hard, irregular granules). Corncob polishes better (softer, more cube-shaped granules).
The "red" media is likely rouged walnut, designed to put a high polish on the brass. The "green" media is probably corn cob with some additives.
Some chemicals and media additives can damage your brass, if used improperly. If the media feels fairly dry, and doesn't cake, it is probably safe to use. But, if it feels gummy, waxy, or easily cakes; It is in the best interest of your brass to throw away the old media, and start fresh (especially the corn cob - it absorbs more additives).
I used to use primarily corn cob media, and only polished with walnut. But, I have used only Lyman's rouged walnut
, for about a year now. It cleans faster, and adds a minor polish; in half the time corn cob media takes just to clean. I run my
fired brass through the walnut media for about 30 minutes. It comes out squeaky-clean. For range pick-up brass, the tumbler will run for 1-3 hours, depending on how much mud is caked on. (It takes more than twice as long in corn cob media. - You can save some time, by pre-washing in dish soap and warm water; but you have to fully dry the cases, before they go into the tumbler. That can take days
I need a new signature. This one sucks.