Pdf Dropdown to textbox fill colours.

I may have imaged it, but i'm sure i've seen a fill in PDF form that had a dropdown menu with squares of colour - Red, Green & No Fill

when a colour is clicked the text box/field fill colour goes red, green or blank.

How is this done, or is it not possible?

i have spent a day or so looking on Google, but i just got confused.

thanks in advance.

Luka