I have a configuration menu with multiple menu, tabs, and some required options. I have asked a similar question regarding this issue, but I have a more specific question. At what point should I show that the required forms are incorrect i.e. red? The menu and tabs can be done in any order and are not required to go sequentially. Additionally, in a scenario where the feature is disabled, the required fields are not required. When it is enabled though, the user must input the correct information. In that case, when the user moves tabs/menus, I have decided to turn the input field red and show an icon, shown in the selected answer to my previous question here. Best user experience for settings menu with required fields?
My question is, when the input fields are blank or incorrect, at what point do I show they are red? Some options I have thought of are
Immediately red until they are filled in correctly. This would make it apparent immediately, but can be irritating if the user is inputting the field and it is constantly changing back and forth from red to normal.
When focus is lost on the input form. This wouldn't work if the user never clicks on the input form in the first place as they wouldn't know anything was wrong.
When the user leaves the tab/menu, they see the warning icon. This makes the user leave the menu and then have to go back and search for where they missed the required form.
Are there any other ways I can achieve this or a combination of options that will best show the user that there is an error? The error can be an empty field, or an incorrectly formatted input.