Richard said:
For all I know, they make a product just for sail slides.

They do, I just haven't been able to find it on searches.

They do make white greases; vaseline is close to white. You can get
the stuff they used to use on car door latches. White stuff in paper
wrapped sticks. Paraffin wax is white. Johnson's paste wax is near
enough white. Just to round out the list of non-staining lubes, soap
is good in dry places such as wooden drawer slides and wooden windows.
Graphite is the one thing that will never come out. Even laundry
marking ink, which is silver, will dissolve in cyanide. Graphite
dissolves in molten iron, which is hard on most sails.

White grease (usually lithium grease)or other greases are not good
choices for the same reasons that Roger found. They also attract and
trap dirt, salt, etc. so I would rather use SailKote or similar dry
lubes after cleaning the track. Now if I could only find that pesky
little device that's made to do that...
Red