Specifying color or multiple colors as an eval argument creates black outlines on the lines.
| eval color = case ( match(app, "dns"), "green", match(app, "tls"), "yellow", 1==1, "red")
i don't have this app installed, so this might not work. i can't find much on the documentation for the app, but it does say
color: The color of the arc in hex format (optional, default "#FF0000") on splunkbase. have you tried using hex values?