Green gastric residuals (GR) are often considered as a sign of feed intolerance and discarded in preterm infants. Probiotics are known to enhance feed tolerance in preterm infants. To assess the composition (primary outcome) and volume of discarded green GRs, and feeding outcomes in extremely preterm (EP) infants in a probiotic trial, composition of pale and dark green GRs in the first two weeks of life from EP infants (<28 weeks) in a randomized controlled trial (RCT: SiMPro) of single vs. three-strain probiotics was assessed. Feeding outcomes included time to full feeds (TFF: 150 mL/kg/day) and duration of parenteral nutrition (PN). EP infants given placebo in our previous probiotic RCT served as the reference group. Analysis involved linear regression modelling with clustered standard errors for repeated measurements. GRs of 74/103 from 39 SiMPro infants (18: single-strain, 21: three-strain) were analyzed. Bile acid content was higher but statistically insignificant (825.79 vs. 338.1 µmol/L; p = 0.12) in dark vs. pale green GRs. Mean (95% confidence interval) fat, nitrogen, and carbohydrate loss in GRs over the study period was 0.02 g (0.01–0.03), 0.011 g (0.009–0.013), and 0.05 g (0.04–0.06), respectively. Overall, SiMPro infants had shorter median TFF (10 vs. 14 days, p = 0.02) and duration of PN (10 vs. 16 days, p = 0.022) compared with control group infants. Z scores for growth parameters at discharge were comparable. Discarding dark green GRs meant higher loss of bile acids during early enteral nutrition in EP infants. Probiotic supplementation was associated with reduced TFF and duration of PN.