How yellow? Is it actually opaque like milk or is it yellow tinged clear fluid?
Colostrum is more yellow colour but it is definitely a milky opaque colour.
Once the milk changes, it's 12 - 24 hours. Double checking with a pH test to see if it's dropped would also be a good indicator.
She’s not foaled yet xThat's pretty normal. Some mares don't dry up fully ever. The fluid that remains is a yellow, clear fluid.
Yes she been scanned and she’s huge right nowI understand, this fluid is not indicative of imminent delivery, in my opinion. Did you confirm pregnancy?