Hi Joe,
The Omnivore KML plugin for Leaflet is used to display KMLs when using Leaflet (and Mapbox), and it only displays blue lines. If you want to change the color of all lines (and not per-line), I’ll see if I can find a workaround for you, but it’ll take some code as it’s not supported by default.
You also have the option of using Google, which does support line colors.