Green or dark grey workouts in calendar

Stupid question and I’m sure already asked, but I can’t find it answered. Why are some workouts marked green afterwards and some dark grey. All the workouts were completed and with sufficient “sufferage”… but are marked different colours. This is using the Android app.

It’s far from keeping me up at night, but curious.

The green ones are made from a training plan. while dark gray is selected from the library.

Also, if you add a workout to your calendar before doing it, it will be green. Workouts you just do straight from Library without adding to calendar - gray as per @jacek.

maybe think about the grey color again as it is known to be associated with disabled interaction?
(enabled/disabled) (completed/not completed)