I am creating a website of Online test in which i want bookmark color

I am creating a website of online test in which a group of button with text 1 to 200. when candidate attempt the ques then button color will change to green . when candidate leave the ques ques then button color change red and a bookmark option for future review of ques then button color change purple. Now i want purple color retain throughout the entire test time untill unmark bookmark . How will i get this? Plz Help