Inertial grids form a family.
Once you have one inertial grid, you can transform it into another inertial grid, with these
kinds of changes (transformations):
displacements
reversals
reorientations
boosts
Any other kind of change, such as accelerations or spinning motions, will result in a grid which is no longer inertial.
The time is indicated by the cycling colors:
green
orange
red
As a mnemonic, they cycle in alphabetical order by name; when the sensor clock is reversed,
the colors cycle in reverse order. This doesn't represent time itself running backwards; it's just that the clocks are running backwards.