What Do The Colours Mean?
The colours that you'll see on your completed trip relate to your average speed.
Yellow = average; green = faster than average, red = slower than average on a sliding scale with the shades indicating by how much. If you tap on the map taking you to a full screen it may be clearer as the average speed is shown at the centre of the colour bar at the bottom of the screen.
Each paddle is standalone - so while you may have had a slower or faster overall paddle, you could have spent more time faster than the average, or had a long break or a period of being stationary.
Equally, you may have had a paddle where your speed maintained around the average (yellow) which is faster than other paddles overall.