It depends if you are after just an override colour in view or a material? Below is the override in view option, I have added some different colour schemes.
If you would like to use actual materials, then you need to have a material parameter added to you families (which they currently don’t have, and then you can use the workflow similar to what is in the link below.
Because my sample project has only your family, it was very easy to select the elements. In a case where there are many Generic Model elements you will have to filter the list using a parameter that can just identify the elements you require. The family name would probably be a good parameter to filter?
Thanks