Make a custom relationship field appear ONLY if another field meets a criteria (Dependent Formula)

I have created a one to many relationship from the Projects Module to the Opportunities Module.

When a user creates an Opportunity, they can now use the new Projects field to link an opportunity to a project. (The new Opportunity now shows up under the custom Opportunities sub panel under the project)

My question is, how can I edit the new Projects field in the Opportunity Module?

For example, I would like to have a drop down field called Opportunity Type, and the options would be: Project, Distributor, Direct.

The new Projects field would ONLY be visible if Project was selected in the aforementioned Opportunity Type drop down.