Praise be to Allaah.
If the situation is as you describe, and you try hard to get
rid of the blood that accompanies the saliva, by rinsing your mouth and
guarding against swallowing any of that, then whatever is left of slight
yellow traces in the saliva does not affect you and your fast is valid, in
sha Allah.
And Allah is the source of strength. May Allah send blessings
and peace upon our Prophet Muhammad and his family and Companions. End
quote.
Standing Committee for Academic Research and Issuing Fatwas
Shaykh ‘Abd al-‘Azeez ibn ‘Abd-Allaah ibn Baaz, Shaykh ‘Abd al-‘Azeez Aal al-Shaykh, Shaykh Saalih al-Fawzaan, Shaykh Bakr Abu Zayd
Fataawa al-Lajnah al-Daa’imah – vol. 2, 9/210.