If you are self shadowing then you will lose the surface shadow effect when it is more transparent( as happens with real fog ). Make the shadow opacity = 1 (the default is 0.5) to provide the maximum shadowing for a given transparency. This may help a little.
Texturing the color and/or opacity can help, although you will need to animate the texture attributes carefully to match the motion of the dust. Alternately you could make the colorMethod = dynamic grid and initialize the grid to a randomized color range.
Another approach is to mix software rendered particles with the fluid. Make "conserve" low and apply the fluid as a field so that they move with the fluid( although a differential where the particles fall a little bit more than the fluid can look natural... this simulates different size suspended particles) Also you may need to increase the volume sample override on the fluid shape to better integrate the render with the particles.
Duncan