There is always a minimum number of guesses you are from completing a travle. This is what travle uses to determine guess 'quality', and which emoji to give to each guess.
If you make a guess which decreases that number, the guess is marked

.
For example, to go from
Spain to
Austria you can go to
France, then via any of
Germany,
Switzerland or
Italy. You're 2 guesses from the answer. If your first guess is
Germany, you're now one guess from the answer - so
Germany is marked

. If you then guess
Italy - even though that country was on the shortest path
to begin with, you're still one guess away. Italy doesn't help you now, so it's marked

instead.

means the path through the newly guessed country is a similar length to the shortest. In the previous example, after guessing
Germany and
Italy, the shortest path is 1 guess. The shortest path which passes through
Italy also only requires one additional guess. Hence, it is marked orange. The rule allows a leeway of 1 extra guess too, so even
Slovakia would still be marked

- the path via
Slovakia requires 2 additional guesses:
Franceand
Poland, which is just 1 more than the shortest path - which only requires
France.

means the path through the newly guessed country is longer than the requirement above.

means there is no path through the newly guessed country. This also means there is no path from the start or end point to the guessed country