This is due to any one (or more) of these issues:

1. The rendering engine (the software that reads and draws (renders) the map on each Garmin devices are different; this can result in some colour differences, or even with some data not displaying on one Garmin device model, but displaying fine on most other Garmin device models. An example of this is when trying to compare the way the map is displayed (rendered) on a modern Garmin handheld device (such as an Oregon, Montana, GPSMap62 or later, etc.) against older models or smartwatches. The very same map will display (render) differently on older devices (such as the Etrex Venture/Legend/Vista models and also the old GPSMap60 models. In this case it is mainly colour changes (the ground in yellow, rather than white), and so on. See the next point for the possible issues with Fenix or Edge models.
2. If you have a device that has Map Themes (such as a Fenix 6, 7, Epix Gen 2, and so on), then as shown here this will affect any map installed; both colours and detail, as that is what Map Themes are supposed to do. This is also true of newer Garmin Edge models (520 Plus, 820, 830, 1000, 1030, etc.) If you set the map to anything other than Classic, it will cause the map colours to change. This is documented here for the Fenix and related models,(along with screen shots showing how the map may be affected) and here for the Edge models.
3. On the latest Garmin Fenix 7 models, Garmin have changed (re-mapped) the colour pallet supported; this is causing lots of strange colour and display issues (even with Garmin's own pre-loaded TopoActive maps). Just take a look at the Garmin Fenix 7 forums to see the amount of annoyance that this change is causing. Why Garmin has not left the supported colour palette the same as it is on the Fenix 5 and 6 models (that support maps) is beyond me! They work pretty well, with only a few issues (some reversed polygon colours), all of which are acceptable.
4. Some Garmin models only support 16 or 64 colours (this is usually the older devices, or most of the smartwatches (such as the Fenix range) that Garmin offer), this will have also mean that the map may display differently, newer handheld devices usually support 256 or more colours and therefore tend to display the maps as intended, with the correct colours.

So, in summary, do not expect the maps to display exactly the same on different Garmin devices, as they all have different processors, functionality and don't all support the same number of colours (or even the same colour palette, at times).

Please Note: As of the beginning of May, 2022, all our maps now use a 64 colour palette instead of a 256 colour palette. This will make the map display more consistently on most devices, as well as improving the contrast, etc. This is documented here with example screen shots.

There are a number of screen shots of how the maps look on specific devices, and in some cases showing how they differ to the normal map display on most Garmin handheld devices, here.