When an activity sync, sometime it is green, so time it is red. What is the significance of the red/green?

Please, someone answer.  I found some announcements about the "colorful new interface" but no definition of what the colors represent or when they change for the same activity.

Thanks for the useful color summary.  In my case the exact same bike activity was red at one time, and less than an hour later was the normal green.  Maybe it was just a short-term glitch.

I did a hike which I expected to be green like all my hikes and cycling. But it was red. I then noticed I had recorded the activity as 'Navigate'. Perhaps you recorded the activity in this way?

I have been told over and over that it has no significance. But ponder with me if you will because my runs typically show up Green. Today my run was yellow and my training changed to unproductive. So my hypothesis is that your activity is a reflection of the benefit to your training. Green, good to go! Orange, unproductive, red, decreasing your fitness. 

Could you post a screenshot of the yellow run so we are talking about the same thing?

This was my yellow/orange run and the training status says unproductive.

I did a test now with different run types and I get orange/yellow for all my runs as well so I think that it is the standard color.

trux wrote orange as a color for trail run in the old post so it is probably the default color for all running activities.

I ponder this also and thought it related to HR zones and this theory seemed to ring true on all my activities until very recently where I stayed on Blue and Grey HR zones whilst doing strength workout and the tile turned up Amber so threw my theory out. Until this point it seemed to hold true. 

activity type doesn’t seem to drive colour as I’ve had same activity return in different colours red does seem to be the ones I’ve maintained most relative effort, cardio workouts/HiiT etc...... 

Could you post a couple of screenshots were there are different colors for the same activity type? I always see the same color for the same activity type and I've done several tests to see if I can get different colors.