Ok, i'll try and explain it [i think this is why]

When you smoke a cig the filter goes yellow with tar. The particles of tar are in the smoke. It'd do the same with weed.

So these yellow particles are in the weed smoke, and the denser it gets the more the particles stick together and give the smoke a yellowish tint.

So it's all to do with the density of the smoke in the bong.

My bongs red though so i cant see it properly - so i cant say it's ever happened to me.