Now it doesn't matter much because high technology printing can pretty much do anything in a jiffy, but in the nineteenth century, it would have been much cheaper if all stamps had been printed in one color. The reason that they weren't was because different letters to different places in differing weights required different postage. This meant that post offices issued stamps with different denominations. The reason that these different denominations were printed in different colors was so that the postal clerks could distinguish the different denominations when they used them. If all stamps were printed in the same color, regardless of denomination, it would have been much more difficult to determine if the correct postage had been paid.
In general, when a stamp was issued the color remained the same, except for minor shade variations, throughout the issuing period. Most stamps are either rare or common depending on how many were issued and saved and how much demand there is for them now. Color comes into play in terms of rarity in three ways