We are blessed with the fact that the world we visualize only has 176 countries. This way, we only need to keep track of 176 indeces. However, you often deal with more data objects so that 256 (as in 0-255) will be used up rather quickly. Luckily, we not only have R but also G and B values and their unique combinations, which gets us to 256*256*256 = 16,777,216 possible indeces you can store. That will bring you far.