This can be done via custom groovy script. To do it:

1. Switch to Administration / Groovy Scripts and define a groovy script say Milestones, with below content:

import io.onedev.server.model.Project

def choices = [:]
for (milestone in Project.get().milestones) {
  if (milestone.closed)
    choices[milestone.name] = "green"
  else 
    choices[milestone.name] = "yellow"
}
return choices

1. Now define your custom issue field Affected Version, specify Available Choices as Evaluate script to get choices, and select script Milestones created above

A minor error in the script. Please use color code instead of name like below:

import io.onedev.server.model.Project

def choices = [:]
for (milestone in Project.get().milestones) {
 if (milestone.closed)
  choices[milestone.name] = "#00ff00"
 else 
  choices[milestone.name] = "#ffff00"
}
return choices

A built-in issue field type will also be added in next minor release

State changed as code fixing the issue is committed

State changed as build #2278 is successful

This can be done via custom groovy script.

That's interesting. Did not know that. Thanks for the tip. And many thanks for the new field type in 6.3.3 😀