> There are no successful economies without blue paint
I don't think that is true. The oldest known mass printed advertising is about 2000 years older than the oldest known blue pigment.
> As far as I'm aware, there hasn't been enough testing to say much about the importance of ads.
I think if you look at some early advertising (e.g. BCE), you'll see that most have a painfully obvious functional form of just simply announcing the existence of a product/service for the world to observe.