Stools can come in several colors usually depending on a combination of what you ate, how it was digested and what juices (enzymes
were used to digest it. Green and yellow are both colors that can occur if you have a lot of bile dumped with your stool, in addition to the other possibilities I mentioned.
There may not be much to worry about, if you don't suffer from any significant pain. The only colour I'd get alarmed by would be red - If red appears on a consistent basis, in spite of eating non-red coloured foods, then I would talk to my GP about it.
As for fibre sources: well I'm not sure why you relate it to fibre sources. If on stopping the use of fibres, you don't see any colours, it could be that your intestine is irritated by the fibre and so dumps more bile, and hence the off-colour stool? Dunno.
But if you really want to know, perhaps you should discuss this with your doc.