This could be due to a number of things, such as:
1. The backlight is set too low.
2. The map has DEM data and DEM shading is enabled (this can make the map seem darker than with this feature turned off)
To enable/disable DEM shading on the actual Garmin device, go to:
Setup -> Map -> Advanced Map Setup -> Shaded Relief: and set it to “Auto” or “Show if Available” to turn it on.
To turn it off, change it to “Do Not Show “
If you don’t have that option, then the Garmin device will probably not support DEM shading, but the DEM data is still there (if the map contains DEM data).
The other possibility is that you have either a Map Theme enabled (Fenix and related watches) or the map set to something other than Classic (Edge 520, 530, 820, 830, 1000, 1030, 1040, etc.) This will change the way the map is designed to look (that is what map themes and overlays do), so simply set the Map Theme to NONE or the Edge map setting to Classic; this will return the map to the detail and colour it was designed to use.