To change the default Color of the Start Menu & Taskbar go to Control Panel > All Control Panel Items > Personalization > Color And Appearance then select Show Color Mixer. These 3 sliders will modify the solid color chosen above that also modifies the color of the Start Menu & Taskbar.
This post is intended for Build 9926 and you are using the default Start Menu.
The Color Intensity slider only modifies the color of the window borders.
Color and Appearance - Change in Windows 10
Color Intensity & Color Mixer functions (Sliders) have been depreciated and no longer have any effect of altering a chosen color for builds newer than 10130.