FrigoSim uses black colouring to indicate that a specific element is incompletely specified.
This applies to both basis components and plant components. Basis components are normally grey-coloured.
When one or more data elements are missing, the colour turns black.
Plant components are normally coloured by the fluid that is contained within and/or is flowing through.
If the corresponding basis component is black, or some initial condition is unspecified, the plant component is presented black.
Black colouring is also used to indicate that a given circuit is not properly specified.
The connectors (if selected to be viewed) turn black.