What a healthy budgie dropping looks like
A healthy dropping is a neat little coil: a firm green or brown solid part, topped with a chalky white or cream cap, plus a small amount of clear liquid. Brown is normal on a pellet diet; green is normal on a seed-and-veg diet. The number of droppings is high — a budgie goes very often, which is completely normal.
Colours and what they can mean
Diet changes colour harmlessly all the time — beetroot and berries can turn droppings red or purple, dark leafy greens make them greener, pellets make them browner. The signs that matter are the ones that aren't explained by food: the white part turning yellow or green (a possible liver sign), black tarry droppings (possible digested blood), or a lot more watery liquid than usual that doesn't settle down.
When it's an emergency
Contact an avian vet straight away if you see black tarry droppings, fresh blood that isn't from red food, very few or no droppings, or bright-green droppings in a bird that has stopped eating. A budgie that isn't eating and pooping normally can go downhill fast.