the Fitbit mobile app
Use this app if you want to stay in particular zone during workout (or even throughout the day). It tracks HR changes and if your HR crosses the threshold of your custom zone you are informed by vibrations and changing screen color of the watch.
There's a different vibration pattern for each zone (lower, target, higher), so it will be easy for you to recognize which threshold you have passed even without looking at your watch. But visual cues are also added for convenience - screen is blue if HR is in lower zone, red if in higher and black with green borders if in target.
It is very easy to set up the custom zone, just click on the corners of watch to increase, decrease min HR on the top and max HR on the bottom. Also you can enable "Always On Screen" in app settings in the phone.