My white text becomes grey in exported mp4 file

I'm editing a video and am using white caption overlays. They all appear white before I export them in the editing tab and preview. However, when I export it to a mp4 file, the file is "incompatible with QuickTime" and all my text overlay becomes gray like the pictures attached. The color grading of the video is fine, only the text colors are affected. How can I fix this? Thanks!