Conditional Formatting To Turn Field Specific Color Based On Two Conditions

I am trying to work with conditional formatting to turn a field a specific color based on two conditions, one of which is variable. The first condition is a list of ongoing events (which can change) and the second condition is a set of known milestones. I want to create a control (check boxes?) that will turn a given field (which has dates in it from another calculation) to either Red, Yellow, or Green.

I was thinking two Dropdown boxes - one for the events and one for the milestones. The user would choose an event, choose the milestone, then choose a checkbox or something to change the color of a field elsewhere in the spreadsheet. Right now, I'm doing this in a very clumsy way by having three checkboxes for each milestone for each event. There are five milestones per event (so 15 checkboxes per event on Sheet2). I will likely have 70+ events so I was hoping for a better way to do this. I've done this for the first three events and need a more efficient way of doing this.

See the attachment : Event Milestones based on Go Date Uploadable.xlsx‎