As best I can make out from some quick messing around, the yellow is apparently used to indicate "not enough wind
to sail" and red to indicate passing over water
shoaler than your draft
. You should be able to adjust those parameters in the settings for your vessel.
It's an interesting app; I might play with the web version a bit more to see how it handles some messier tidal cases.
As to safety
, I suggest caution when using do-it-for-you apps for navigation
. One should never assume there's anything "safe" about an auto-computed route.
This looks like a useful tool for incorporating weather
and tide, but ultimately you are responsible for the route you sail. Your evaluation of that route should be done using the best available data; that can certainly include consulting 3rd party charts
as a 2nd opinion, but should always include up-to-date official charts