Require a specific textbox to be filled only if a specific answer from a listbox is chosen

so i have a listbox right now and it has 5 choices. I want a textbox to require an answer but ONLY IF certain choices from the said listbox is chosen as an answer.

example:
Listbox is about favorite color, and choices are:
1. Red
2. Orange
3. Yellow
4. Green
5. Others

If the user chooses either 'Orange' or 'Others', a textbox should require the user to fill it up - a message box should appear; otherwise, if choices 'Red', 'Yellow', and 'Green' are selected, the textbox does NOT prompt the user to answer it.

***i am very sorry, i forgot to mention that this is a userform from excel.