It sounds like you accidentally changed the theme/colors for windows.
If you are on windows 7: https://www.howtogeek.com/howto/21161/c ... -software/
If you are on windows 8: https://www.isunshare.com/windows-8/cha ... puter.html
If you are on windows 10: http://www.intowindows.com/how-to-chang ... indows-10/
If you are on a different version of windows; just search 'change taskbar color X' with X being your version of windows
If for some reason this doesnt work, or you want your title-bars green, just not your taskbar/start menu then classic shell can help:
Right-click the start button and select 'settings
then in the searchbox type 'color'
check "override glass color"
and click the text field then the "..." icon to pick a color (this wont necessarily be the exact color of your start menu, (because the skin itself decides how much of the glass color to use) but it should remove the green.
Then click the 'taskbar color:" option and pick a color.