Add the following code to Customizer > Custom CSS, and change the colour code accordingly:
.single-topic #bbpress-forums .hentry.author-role-keymaster, .single-topic #bbpress-forums .hentry.author-role-moderator, #bbpress-forums .bbp-search-results .hentry.author-role-keymaster, #bbpress-forums .bbp-search-results .hentry.author-role-moderator {
border-color: #9538c5;
}