I thought Türen also meant gates? But it didn't accept that... Does Türen only mean doors?
It's close, but not quite. The door is die Tür; the gate is das Tor. Gates, meanwhile, is die Tore.
(Just be careful not to say "der Tor" instead of "das" -- "der Tor" is the fool!)
So did I for a minute, and I thought of gold for gelb, which evoked the Golden Gate Bridge, although I'm not sure what Nur zwei Tore sind gelb with regard to said bridge (which I think isn't actually golden colored) would mean.
"Nur eine Tür ist gelb." You just use the indefinite article if it's "one" as in a preceding adjective like that, and the statement would translate as either "only a door is yellow," or "only one door is yellow."