Hi all,
Just wanted to let you know I was struggling with this real badly. I am making a video that needed to have the same color background as a web page and using the same hex just wasn't working. It looked really amateur with a 'really close but not the same purple' background to the video.
I had an After Effects comp dynamically linked into Premiere (for touching up sound) and was rendering on AME. I used all the tips above, changed rendering settings to max bit rate, etc. Nothing worked. Tried all settings. However, one person mentioned above that it renders more accurately in Premiere. Rendering in Premiere gave me the exact same color (you can see a test image with the background and video flush here).
I hope this can help someone else at some point.