I want to ask, why my colour accent doesn't change as the background changes?

I have my settings turned on already.

It does change actually, when I turned the setting off and on again. But when the background changes, the accent doesn't change. Is it a bug? Do you have any workaround to make it automatically changes?

in windows 8.1 it changes according to the background.

This might have caused due to any updates or due to video/graphics driver compatibility issues. I suggest you to try the following methods and check if it works for you.

Method 1:

Try installing all the updates in Windows Updates and check if the issue persists. This is to check if the issue is caused due to any update related issues.

Method 2:

Try reinstalling the Video/Graphics card drivers in compatibility mode and check if it helps. This is to check if the issue is caused due to driver compatibility issues.

Refer to the following Windows 8.1 article to install the driver in compatibility mode. The steps for Windows 8.1 applies to Windows 10 as well.

http://windows.microsoft.com/en-US/windows-8/older-programs-compatible-version-windows