In fact, the description can be improved.
But perhaps what should be changed is the interface.
It is strange that the type of property is defined by the way its value is written (1.0 for float or [1.0, 2.0, 3.0] for vector).
Perhaps the type of property should be defined by an Enum, (as is done for the subtype).

There already is a task for the custom properties design, T73814. So removing the UI tag.

The documentation does not explain the way to add a custom property subtype, such as a color.

In the docs, with screenshots:

Property Value > Vector - [1, 2, 3], [1.0, 2.0, 3.0]

Subtype - For vector properties, specifies the type of data the property contains, which affects how it appears in the UI. Most useful for color properties.

https://docs.blender.org/manual/en/dev/files/data_blocks.html#editing-properties

Isn't that it?

Isn't that it?

I stumbled upon that page knowing it was possible to use colors as custom properties, yet I couldn't figure out how to add them based on that information (eventually found out how to do it from an ages old YouTube video). There are a lot of mental hoops to jump through if you are not intimately familiair with Blenders internals. This includes that a vector can be a color, how to properly format a vector in text (you don't have to do that anywhere in Blender normally), that the subtype dropdown is not missing from your version of Blender but only appears after you made it a vector and that you have to add a fourth vector value to add an alpha channel. Even then when you enter [1,1,1] (which is mentioned as an example of a vector) the subtype does not even appear, it only appears if you add a decimal to each value. So I think there is a lot of room for improvement, and of course the most obvious thing would be to improve the UI, but I guess that is unlikely to happen this short before the release of 2.91.