I assume you can’t have had it long if you don’t know what’s on it. It’s much the same as with six string guitars; bronze strings are less “tinny” than grey looking ones and heavier strings give a more solid sound than lights. You might like to try some of the coated strings which tend not to be as bright as their un-coated equivalents.

The other thing to bear in mind is that twelve strings aren’t usually tuned up to the same pitch as six string guitars. Tuning a tone, or even two, below concert pich will give a deeper sound.