With the Garmin Inland Lakes charts, I believe that those are "Markers" or buoys. They aren't shown as red or green as I think they aren't really true "navigation" markers, but rather more a "milage" marker indicating if you are moving up a lake or down a lake.
You should see some of those black "dots" as well as some buoys indicated with the actual red or green colors depending on which lake you are looking at.
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