In the power, cadence and heart rate data box at the top left…
The heart symbol and ‘BPM’ text could change colour to show what heart rate zone you are in. At the minute, I look at my watch for this info (bit tricky when on the aero bars and rollers!), so having it right there on the screen would be really helpful.
I just came to make the same suggestion. I’m a little concerned reading yellow or green text may be hard for some folks. I suggest instead they change the color of the heart picture itself, not the actual BPM value.