Rendered video colour looks different to comp window.

Im trying to render an mp4 to be used on a website with a specific BG colour.

If i create a solid with the BG colour in AE it looks fine in the comp window. Once exported the bG color is slightly different, ruining the illusion of the mp4 being sat on the BG on the website.

ANy ideas how to fix this?

The attached image shows the original BG colour on the left, and rthe rendered colour on the right