I answered "Is your favourite colour yellow?" and it wasn't accepted.
The only thing I can see different is my spelling of colour, as opposed to the US spelling color.
It should be accepted. If you encounter the same problem again, take a screenshot and file a bug report here.
I answered "Is it your preferred color yellow?" and it was not accepted.
Why would you put the "it" there? I do not think it is correct.
I look at this simple sentence with very little familiarity. Czech is so different! (One week so far learning it.)
Color is spelt colour in Australia, think it should be corrected and accepted! Unfair to be failed for it.
Brittish spelling is accepted automatically by Duolingo software. If you think it is buggy, report a bug to them.
The people responsible for the software certainly do not read it here.