Hi Robert,
The signal chain shows each step in the audio path and the light shows the quality of the chain. Yellow is low, green is high and purple is bit perfect. The chain is only as good as the lowest quality component in it.
Not all Tidal tracks are lossless. When the quality of the Tidal track changes, Roon is letting you know. It’s not anything you have done.
Edit: I’m not a Mac user so can’t assist with the 24 bit conversion question. A file recorded in 24 bit can have a greater dynamic range than a 16 bit recording, but converting from 16 to 24 bits won’t add dynamic range to the recording. It is done for compatibility reasons rather than to improve SQ.