If you are drinking your morning coffee and having a bite with it I do apologise for what you are about to read may put you off slightly. However, here at DS we like to make sure you are well informed on what exactly is happening when you go to the toilet and 'drop the kids off at the pool' so to speak :)
So just like the rainbow your stools come in different colours and it all depends on what you are eating and how you are feeling. Green, Yellow, Black, Red and other colours are all signs of different things happening within your body. For example, a greeny colour can indicate that your food has been digested too quickly. Alternatively, a reddy colour can indicate you have eaten something rich in this colour, for instance beetroot!
Reference - Daily Mail - http://www.dailymail.co.uk/health/article-2611166/What-does-poo-say-YOU-Colour-guide-reveals-health-clues-stools.html