How to make a shape the same colour as the background

[Daniel29200184oq05]

Hello friends, I wanted to try to remake the fall guys loading screen and I wanted to make the circle the same colour as the background as it expands from the centre of the screen. I also don't want the background to be visible through the circle. I've tried different blending modes and layer styles but I none of them were what I wanted :p. 

I am very new to AE but thank you for the help in advance.

[Rick Gerard]

Use a shape layer with a fill and stroke. It looks like you are using a mask with a Stroke or just a shape layer with a stroke only. 

When you ask for help, it is very helpful if you show us your timeline so we can see where you are in the process.