seems it sounds like Bile , (stomach acids) , was it after a feed ?? or after a break from feeding ... it it continues go to the docs .. it may just be a one off thing and she burped on an empty tummy and brought up bile ??

you know how somethimes we throw up in our mouths ???(eww i know but it happenss) thats bile ..
if it is bile and a one off -nothing to worry about ..
just keep an eye on her .... how is her behaviour??