Comic Strips

Ooh, I like this one! I think it's for the same reason that liquids like egg whites, sea water, and even some broths can be clear when at rest but opaque when frothy. And the reason for that is the same as the reason why a straw in a glass of still water looks like it's broken: refraction!

So, when light enters or leaves a medium at an angle, it is bent by the transition between the medium it's entering and what it's leaving. And it's not a lossless process; some of the light that reaches the boundary is reflected instead by the medium, some is reflected internally, and some is absorbed by the medium.

With a single large bubble, you wouldn't notice this; the boundary is tiny, there's way more of one medium (air) than the other (soap), and the bubble's shape means that the light actually tends to bend back to its original trajectory again coming out. But if there are many, many thousands of tiny bubbles, with pretty similar amounts of soap and air, some sharing boundary layers so that light can enter and leave at different angles, reflecting light all over the place and refracting it from everywhere, you're just going to get that reflected mess.

And even if the soap is dyed, that wimpy little amount of dye isn't filtering the color of the light enough for it to overwhelm the combined color of all the other light in the room; which, in most cases, basically average out to white.