While high reaching opaque clouds are depicted in red, mid-level clouds are shown in ocher colours in the Dust RGB. These clouds are mainly water clouds. Ocher is a mixture of the red and green colour beam, so both BTD are involved. The blue colour beam does not contribute as these clouds have a cold cloud top temperature.
IR8.7 is able to discern ice from water droplets. This does not mean that ocher clouds in the Dust RGB do not contain ice crystals, but water droplets prevail.
For very low and warm water clouds (e.g. over the sea), the blue colour beam becomes stronger and the Dust RGB depicts these clouds in magenta colours (see image below).