Hypertrophic pyloric stenosis should be high on your differential with the history of 'projectile vomiting'. Other considerations include intestinal atresia, gastroesophageal reflux and gastroenteritis.
Ultrasound of the pylorus should be the next imaging study.
Bilious vomiting implies there is an obstruction beyond (distal to) the ampulla of Vater (where bilious secretions from the liver enter the GI tract).
When emesis is bilious (dark green in color), the differential diagnosis changes to include malrotation with midgut volvulus, annular pancreas, duodenal or jejunal atresia, and duodenal web among other possibilities.
In the setting of bilious vomiting, an urgent upper GI (fluoroscopic study) would be the first step in radiologic evaluation.