Dietary Influence: A change in diet, such as consuming foods or drinks that can alter stool color (e.g., beets, blueberries, spinach), is a common and harmless cause of differently colored excrement. Given the patient's age and the absence of other symptoms, this is a plausible explanation.

Gastrointestinal Infection: Infections like giardiasis or viral gastroenteritis can cause changes in stool color and consistency. Although the patient had a normal gastro/coloscopy four years ago, infections can occur at any time.
Food Intolerance: Lactose intolerance, fructose malabsorption, or other food intolerances can lead to changes in stool color due to malabsorption of certain nutrients.
Medication Side Effect: Certain medications, such as iron supplements or bismuth subsalicylate (Pepto-Bismol), can alter stool color.

Gastrointestinal Bleeding: Although less likely given the patient's age and previous normal gastro/coloscopy, gastrointestinal bleeding (e.g., from ulcers, inflammatory bowel disease, or vascular malformations) can cause black, tarry stools (melena) or red stools, depending on the location and severity of the bleed. This condition requires prompt medical attention.
Pancreatic Insufficiency: Conditions like chronic pancreatitis or pancreatic cancer can lead to malabsorption of fats, resulting in pale, greasy stools. While less common in a 25-year-old, it's crucial not to miss these diagnoses due to their significant implications.

Wilson's Disease: A genetic disorder leading to copper accumulation in the body, which can cause green or blue discoloration of the stool due to copper oxidation.
Porphyrias: A group of rare genetic disorders that can affect the production of heme, leading to the accumulation of porphyrins, which can cause stool to appear red or purple.
Celiac Disease: An autoimmune reaction to gluten, which can lead to malabsorption and changes in stool color or consistency, although this is more commonly associated with other gastrointestinal symptoms.