The button colours are determined by this CSS in Content Views v2.0.2
.pt-cv-wrapper .btn-success { Your preferences in here for normal view }
.pt-cv-wrapper .btn-success:hover { Your preferences for mouse cursor hover }
.pt-cv-wrapper .btn-success:visited { Your preferences in here after visit}
Try this in the CSS customisation section of your theme to change the read more button text color after the link has been visited
.pt-cv-wrapper .btn-success:visited {color:#fff;background-color:#000}
That’s if you wanted the text white and background black after a visit
Hope this helps
Neil
N.B. For info, I did have to amend this slightly as follows to get it to work:
.pt-cv-wrapper .btn-success:visited {color:#ffffff !important;background-color:#8c0f17
Thanks again.