Is the yellow color remaining on the tissue after passing gas impure, requiring istinja’ before every prayer, or can it be ignored and not affect wudu’ and prayer?

If the color present is that of an impurity (najasa), then it is impure and the body must be purified from it before prayer. Wudu is not valid before its removal according to many jurists. The persistence of the color is only excused if its removal is difficult.