Multi-select text to IDs

Hi 

I have a many to many relationship which I want to simplify by making use of multi-select text fields for the user to select from. But the challenge I have is that whilst I want my users to be able to select text items from a list, I need QB to convert these text selections to a list of RIDs for use in my third party app (via API).

So imagine I my multiselect gets its values from a shared field in another table, with the following rows (RID; Value)

1; Red
2; Blue
3; Green
4; Purple
5; Black
6; Yellow

When the user selects 'Red;Green;Yellow' in the multiselect field, I want QB to translate this (dynamically) to a list based upon the IDs.

So in this example, I'd want the result to be '1;3;6'

It don't need the returned IDs to be in any specific order.

Any ideas appreciated.

Thanks

David

PS Obviously this is easy to do by 'hard coding' the values into an if or case statement, but I really want this to work dynamically and be 100% data driven, so no maintenance is required.