Why does oil change the color of salmorejo?
When tomatoes are fried, you can see that the oil becomes coloured red - so at least some of the colour components are preferentially soluble in oil.

I will hazard a guess that, under the circumstances mentioned, the oil coats the tomato particles, and some of the colour migrates into it to give a red surface. Because the oil forms a coating, that coat will inhibit oxidation.  The oil will also improve the mouth feel of the soup.

Just the way I see it ...