Curious what your workout graphs should look like? Accuracy first and foremost depends on having your correct Max HR set in your profile (see How Can I Check if my Estimated Max HR is Correct?).
Workout Elements
The colors on your graph relate to the stress placed on the cardiovascular system and are going to largely correlate to these elements of your workout:
- Resistance
- Speed
- Was the exercise static (a plank) or dynamic (burpees)?
- Muscle groups you worked
- Number of reps
- Body position
- Rest interval
Examples
For example, a yoga class may feel and be very intense, but it may not increase your heart rate in the same way a cardio workout would. In this case, a graph that displays grey and blue zones would be expected.
An interval workout may display peaks and valleys indicating a high-intensity work phase and a low or moderate-intensity recovery phase.
A resistance training workout may look surprisingly low-intensity despite the hard effort exerted during the workout. You might see some warm color bars or green here and there throughout the graph.
A steady-state cardio workout, such as a bike ride or jog, might have big blocks of one color representing the the sustained effort level, with cooler colors to start end end for the warm up and cool down. 
When it comes to the red zone, more is not more! The fuel your body uses to power you through this level is fast-burning and in limited supply. That means it is good for short burst of activity and is not a zone you would train in every day. The interval workout above is great use of the red zone.
Errors: Spikes, Drops, Freezes
If you see a graph with sudden spikes, sudden drops to light grey in the middle of an exercise, or freezing or "Searching," this means your device is struggling to maintain a clear reading of your heart rate.
Please see our troubleshooting guides:
Why Won't my MZ-Switch Connect?