The reason that clouds look, well... cloudy, is because of scattering. Specifically, the type of scattering you observe is called Mie scattering. This type of scattering occurs when the refractive index of a medium varies on a length scale comparable to the wavelength of the light (on the order hundreds of nanometers). In the case of clouds, this variation is caused by the formation of water droplets (water has n=1.3) in a background of air (n~1).

It the case of liquid mixtures, something very similar happens. Take a look at this image showing two liquids that were shaken up in a funnel. Notice that both the top and bottom liquids look pretty clear. This is not surprising since the liquids are homogenous and the refractive index is more or less constant. As a result you get little scattering. In contrast, in the middle where the two liquids mix you have a messy patchwork of droplets of one liquid suspended in the other. This spatial inhomogeneity creates scattering centers, which in turn makes the region look cloudy.