Amoled devices such as the Venu family, MARQ & Epix Gen2, Forerunner 965 and 265(s) and some other devices support over 65,000 colors. However, Memory-In-Pixel screens devices such as Approach, Fenix 7 (and lower) family, most D2 devices, Descent and most Forerunner devices only support 64 or even 16 colors. For screens supporting 64 colors, the supported color codes can be found in the table further in this page. (The full overview of devices can be found on the Garmin Compatible Devices page.)
For Amoled devices you can use the same table below or this page to pick and choose color codes if you want to use more shades of the same or other colors than the once in the table. When using a color code in watch face settings, always make sure it starts with 0x, e.g. 0x00ff55.