An option group, by definition, allows one option to be selected.
If you need to allow for multiple options to be selected, you need to create
a related table to store the options for the record in the main form. You
can select multiple options on different rows of the subform, typically by
using a combo box.
Example:
Client table (one record per client):
ClientID primary key
Surname
...
Color table (one record per color name):
ColorID primary key
ClientColor table (one record for each color a client likes):
ClientID which client this record is about.
ColorID which color this client likes.
This one-to-many set of relations is the very core of how relational
databases work. For another example, see:
Relationships between Tables (School Grades example)
at:
http://allenbrowne.com/casu-06.html