Issue with the "Intensity" of the Highlight Colours ..

Hello Adobe Community,

Can anyone please explain to me why the "Intensity" of the Highlight Colours is diminishing - to the point were it is hard to see it on the Drawing / PDF. (and the corresponding print out)

I use the Highlight Tool to "mark-up" specific things in Parts Manuals, but tonight (as I continues to do this) I noticed that the colours have become alot lighter.  Please see the attached files.

Can anyone please explain to me why this is happening, and how I can get back to the original Intensity?

Thanks in advance.

Kindest Regards,

James