The value "0" is red and the value "1" is green, the others blue I think. That is done somewhere in the template CSS.

The styling itself comes from the edit class "btn-group" which is applied by default. If you change that to something else you get regular radio fields. Unfortunately removing the class doesn't work as it is specified as a default value which will come back next time you edit it.
We may remove that default value so by default it is a real radio if itis more helpful. I think that default value is there so the radios look like most other radios in the form.

Well, i like the design very much - just the User might expect something different. Please don´t remove the class. .... But: Can it be overridden somewhere by the user if he doesn´t use BS in Frontend?

The class is specified in the field. Look at the "Options" tab. There is an "Edit Class" which by default contains "btn-group". This is the class used in the forms. The user can adjust that class to his needs.

3rd button is blue in Fields (move Field).

To get 3rd Button blue template.css of protostar must be changed?

I think that comes from the template.css, yes.

@coolcat-creations let Issue open?

I am closing this as it appears to have been answered. If that is not correct it can always be re-opened