Normal variation due to formula change: A recent change in formula can cause variations in stool color, including grey, due to differences in how the infant's gut processes the new formula components. This is often a benign and self-limiting condition.

Gastrointestinal infection: Viral or bacterial infections can cause changes in stool color and consistency. Grey stool could be a sign of an infection affecting the gut.
Intolerance to the new formula: If the infant is intolerant to components of the new formula, it could lead to gastrointestinal symptoms, including changes in stool color.
Constipation: Although less common with a change to a formula that might be easier to digest, constipation can sometimes present with changes in stool color due to the concentration of bile pigments.

Biliary atresia: A condition where the bile ducts are blocked, leading to a build-up of bile in the liver. It can cause pale or clay-colored (acholic) stools. Early diagnosis is crucial for effective treatment.
Intestinal obstruction: Any condition that partially or completely blocks the intestine can cause changes in stool, including color. This is a medical emergency.
Gastrointestinal bleeding (upper): Although more typically associated with black, tarry stools (melena), in some cases, especially if the bleeding is slow or intermittent, stools might appear grey due to the partial digestion of blood.

Congenital infections (e.g., CMV, toxoplasmosis): These can affect the gastrointestinal system and cause changes in stool, among other symptoms.
Metabolic disorders (e.g., tyrosinemia): Certain metabolic disorders can affect the liver's ability to process bile, leading to changes in stool color.
Gastrointestinal anomalies (e.g., intestinal aganglionosis): Conditions affecting the structure or function of the gastrointestinal tract can cause a variety of symptoms, including changes in stool color.