As said Laure in comment, they are pronounced in exactly the same way.

You can’t distinguish if your are talking about one or many colours.

In this particular case, they are pronounced the same way, because the first letters of magnifique and couleur (respectively m and c) are consonants.

But in this example, you hear the difference:

Quelle étrange abeille !

Quelles étranges abeilles !

Especially because the firsts letters of "étrange" and "abeille" (respectively e and a) are vowels. In French, we make the liaison between a consonant and a vowel. So you hear [z] for the two first ‘s’.

Have a look here to understand more about liaison: http://www.spellandsound.com/french-liaisons/