Yes, it's invariable in writing and may be so in speech too. But in speech it can also take the -a or -t.
min klanning ar apelsin, if you do not want orange for the fruit, why do you want it for the colour ?
in Icelandic it is appelsína for the fruit and appelsínugult for the colour.
Why does the ä in klänning (and färgglad) sound different from the ä in är (and färg)? Is it something that can only be learnt from memorising every word?
Orange can take the tje or sje sound depending on the dialect. In the kind of Swedish we teach, the former is more common.
No, not really. In spoken Swedish, it's treated like a regular adjective that gains -t or -a when applicable, but it's invariable in written Swedish.
Now I wish I had learned the swedish word for orange before it in english. It took me forever to learn to spell it and now it sounds like it should be spelt. Jag älskar svenska