H264 mpg whites look more grey than prores .mov

I have been working on a long video edit which has a lot of graphics on white backgrounds.

When exporting videos I get into the habit of using maximum colour depth and use maximum rendering quality.  With a very long video, I often bring the target rate down to 4. 

However, when I use the maximum colour depth option, the whites are becoming much more grey, than the prores in the final mpg. I checked the whites in after-effects with the pipette of the original footage and it looks like solid white. (As I have found that if there is a bit of colour, even 1% it can be re-interpreted as grey. However, this is not the case)

I am not sure why this is happening, is there anything else which might cause this issue?

Thank you 

Best regards

Julian