In Time Machine, what is the difference between pure red bars for backup dates, and dull red bars?

I am attempting to restore an old file from one of my backup drives. The backup drive has been in frequent use since 2017; I want a file from that year. However, when I go into Time Machine, it shows backups from recent weeks with solid red bars underneath the dates presented in a vertical row along the right side of the screen. It shows older backups with dull red bars, and does not respond to my attempts to recall those backups. I am supposed to know that dull red bars indicate files that are backed up but are not accessible by me? Or that dull red bars indicate that Time Machine will need some time to recover those files? Or that Apple changed the format of files for Time Machine at some point without providing any means of translating old files? I am unable to find any documentation on Time Machine that explains the difference between dull red bars and bright red bars.