Not Isempty ( FilterValues ( List ( "red" ; "blue" ; "green" ; "yellow" ; "orange" ; "gray" ) ; $choice ) )
will be true if the values in $Choice also exist in the listed values.
Note that the list of values I have created using the list function can be created in a number of different ways. ValueList items can produce such a list, listing all the values in a defined value list. A text field formatted with checkboxes can be used in place of the list function and then you are checking $choice against the list of values selected in that check box field.
Thanks Phil for your insight and for the flexibility of your examples.