If you whip Marmite, it becomes full of tiny bubbles - the more you whip in the more bubbles are crammed in.
Because Marmite is so sticky and viscous, the bubbles can hold their shape for a long time.
The reason the colour change occurs because as light travels into or out of an air bubble, it changes direction - the more bubbles the more light is scattered rather than be absorbed.
Therefore, the brown viscous Marmite will have had less opportunity to absorb the light, and what comes out is a a mix of Marmite and white light - therefore the Marmite is visibly lighter with air bubbles in.
In short, the more air is whipped in, the whiter it looks - much like egg whites.