Why does the moon appear yellowish after sunset?

It doesn't, so "why" is a moot point.

The moon may appear yellowish or even reddish when it's near the horizon, for the same reason the Sun does: the light has to travel through more atmosphere, which means the shorter (blue and violet) wavelengths have more opportunity to get scattered out. The longer wavelengths that don't get scattered but reach your eye directly give the sun or moon that yellowish or reddish cast.

The color the moon appears to be is affected almost solely by its position; whether the sun is rising, setting, or on the opposite side of the world has essentially nothing to do with it.