It is likely dietary. Though you say you have not changed your diet, it could be something as simple as a vegetable, fruit drink, or such. Also the are many non-soluble fibrous foods we eat. These do not break down totally with chewing and digestion. Therefore pieces of food will appear in stools. Examples are skin from tomatoes, corn, leafy vegetables.
If it were me, I wouldn't be concerned at this point by the color change. If however, something else changed, like the consistency, frequency of stools, or the development of abdominal pain, then I would consult my physician.
Moderator Crohn's disease, Osteoarthritis and Irritable Bowel Forums
CD, Ankylosing Spondylitis, small fiber peripheral neuropathy, avascular necrosis, peripheral artery disease, pulmonary hypertension, degenerative disc disease, asthma, severe allergy and a host of other medical problems.