how to save a textbox val if I select an other option in DD

Question

How to save a textbox values if I select an other option in Drop-Down field

 

suppose I've a 3 options in a drop down box say red , blue, others. If a user select option as an others then below a text box should be visible to wrtie his own favourite color. how I can populate colors (textbox) in th drop-down column