Hello @abangfairul,

As much as I’d love to help you but as you want to change the basic color of overall theme, currently the free version of the theme doesn’t have option for color customization.

But advanced color option is available in pro version of this theme, with more and more advanced option for the customization of your site through easy few steps through customization section.

If you are willing to buy pro version of the theme then there is a lot color option available for you.
You can check features of pro version of the theme here http://themepalace.com/downloads/education-hub-pro/

But as seen in you topic if you only want to change the menu color then you can do with simple custom CSS.
For this you need to go to Admin Panel / Appearance / Customize / Theme Options / Advanced Options.
In this section you need to add below custom CSS.

#main-nav {
    background-color: #5CB85C;
}
.main-navigation ul ul {
    background: #5CB85C none repeat scroll 0 0;  
}

You can change the HEX value of the color to the color of your choice.

Hope this will help you.
Let me know if there’s anything else I can do for you.
I’m happy to help.

Best Regards!!!

In premium version of this theme, color options are distinctly provided to give the flexibility of changing the color for different available sections and elements.

For more details, check the colors section in our documentation:

http://themepalace.com/theme-instructions/education-hub-pro/

If there are any further query, then, please feel free to post them.

We will really appreciate if you could help us too by rating our theme in WordPress repo here:-
https://wordpress.org/support/view/theme-reviews/education-hub

Thanks.