These are darker or lighter patches or bands of single colours due to mineral localization, in black this occurs as "oil patches" -patches which look as if oil has been spilled. These are actually permanent patches, After laying and seeing from different angles, it is easy to see which pieces have these defects. Tapping With your finger nails or spoon may give different sounds on these patches compared to rest of the granite.

 

There are color variation for some natural stones.