A quart of wheat for a day's wages, and three quarts of barley for a day's wages, and do not harm the oil and the wine. This is symbolising a time of partial famine and want, when some items, especially those essential to life and part of the staple diet, like grain for making bread, will be exorbitantly priced. People spend a whole day's wage just for enough to make a loaf of wheat bread, or a few loaves of barley bread, but the luxury items like oil and wine are unaffected. It's the colour of death, because it's the colour of a corpse. The rider here is explicitly named as Death, with Hades following behind.
Death is the result of the previous two riders' work.
I don't know what the reference to the wild beasts of the earth means, but in general terms this seal is talking about those who are killed as a result of violence and famine. Let me first of all say they are not prophecies regarding events in the future. They are images, symbols, of what is happening on earth throughout history. Well, numbers are very important in Revelation, and this episode is brought to you by the numbers 4 and 7. Seven, as we have seen previously, is the number of perfection or completion.