Putty title bar color

None of the above. PuTTY has always used a standard Win32 window, where the title bar's color depends entirely on your current Windows theme and not on PuTTY settings.

If you are using Windows 10, go to Settings → Personalization → Colors and select the option "Show accent color on: Title bars". This will make the active PuTTY window have the title bar in whichever color you have selected in the same settings page.

(This generally won't affect UWP apps, such as Settings itself, and will have varying effects on Win32 apps which draw their own titlebars, such as Chrome or Firefox.)