While primarily the restriction for Baltic Reindeer relies on the edit list there is also a smaller secondary restriction with color ranges and the essence of semi-realism. Since reindeer are not genetics based like huskies, they do not utilize specific color or marking genetics in the way huskies do. This means reindeer can be in any pattern you would like, aside from changes like albinism or melanism that completely overwrite any colors and patterns.
While reindeer are allowed to be unrealistic in pattern, the species still drives to ultimately create designs that are semi-realistic in color. On a very general, vague level, this means sticking to earthy colors you would see in a cat or dog and avoiding super bright colors. It also means not using color schemes like green or brighter yellows.
The easiest way to interpret this is to determine how saturated (and by extension, how bright) a color is. Below is a general guide of acceptable ranges within Baltic Reindeer.
There are of course, with all things, exceptions to these rules. For example, a brown that has a copper-green tint. The best way to work about this rule is to use a muted version of the color. Below are some color palettes that would fit the bill for semi-realism (with some being flat out realistic), using the coolors random generator.
To compare, these would be unacceptable color palettes. As you can tell, some of these colors share similar hues to the acceptable ranges. However, these are brighter and a lot more "pure" in their color, resulting in a very rich hue. These would not be acceptable standalone.
Of course, muted colors are not the only way of designing. Since semi-realistic and "bright" colors can be super subjective, the best way is to simply just make sure these colors are not too "eye blinding". The main exception to this is the usage of more saturated colors as minimal accents.
But not like this, because they are way too saturated as a base:
While they could not be used as a base as is due to the saturation, these are still acceptable and meet guidelines because the design they are on only utilizes them in small areas, and mainly as an accent. In these instances, these brighter saturated colors abide by the rules because they are a minimal part of the design and there are other colors being utilized as the main design.
Ultimately, these guides are super objective! Therefore, when we refer to these rules we generally just abide by not using eye-blinding colors and it allows us to meet in the middle.