Yes, definitely go to a doctor. The NHS guide says go to the GP or call 111 if it’s yellow: https://www.nhs.uk/conditions/reflux-in-babies/
Do you feed formula and breastmilk? My boy had this when I gave him formula after breast. Deffo speak to a medical professional to be safe but don't worry too much!
Call 111. My little boy has just came out of hospital after 2 nights with gastroenteritis and it was this colour
I had this with my little one, called 111 they sent me to A&E so they could do blood test. Luckily nothing wrong but better to be safe!
When my LO was 3 weeks old he was sick this colour and 111 advised us to go to A&E, they observed him and we were told the colour was because it was milk that had started to digest/ mixed with stomach acid and that it is probably reflux, he’s 12 weeks now and is absolutely fine except suffering from a few reflux symptoms still, deffo ring 111, even if its nothing its better to be checked out as this can also be a sign of other things as mentioned above x
I think yellow is the color we were told to go to a&e with when our girl was a few weeks, but it could also be reflux