"My favourite colour is blue."
Translation:Мой любимый цвет — синий.
Голубой is a different shade of blue, but yes, this should be accepted too.
Си́ний is a darker shade of blue, and голубо́й is the light blue (голубо́й also includes some part of 'cyan'/'turquoise', but not the greenish cyan). For example, the Duolingo interface is голубо́й for me, but Facebook interface is си́ний.
А люпи́н си́ний:
Вот си́нее мо́ре и голубо́е не́бо:
А здесь, наоборо́т, си́нее не́бо и голубо́е мо́ре:
I'm guessing this would change the emphasis, but I wrote:
синий - это мой любимый цвет
Is this an appropriate translation?
I think the difference is almost the same as between "Blue is my faviourite colour" vs. "My favourite colour is blue". This depends on what's your sentence about. If you start with blue, then you tell a new information about the colour blue: the new information is that it's your favourite colour. If you start with 'my favourite colour', then you want to tell new information about yourself, that your favourite colour is blue.
I think, in this sentence, word order works similarly in English and in Russian.
This is a secondary meaning. Just like the English 'blue' can metaphorically mean 'sad', the Russian 'голубо́й' can mean 'gay (male)'.