Hi,
I looked more closely at the KML file on your site and I think it won’t display the right color line in Google Maps either. I’m not a KML expert, but I think there’s some incompatibility between Google Earth projects exported as KML and the Google Maps API. It’s something to do with the StyleMap tag. Google has several issues for this in their bug tracker. For example:
https://issuetracker.google.com/issues/35816676
If you’d like to email me directly, I’ll respond with a “corrected” version of the file that displays as a red line.
Also, I’ve updated MapPress to make the Omnivore plugin display the correct line colors when using Leaflet. If you’d like to try the new version please let me know.