Help - guest at Catholic wedding?

The only colors that don't go with dark navy are brown, black and also that bright shade of blue that's usually called royal blue. Paler tints of the navy are very pretty (like pale pastel blue, sky blue, etc). And then pretty much any other color you want. Also, it's a really pretty dress, but doesn't lend itself to a shawl or cardigan, so get a sweater you'd like and want to wear again. Don't try to match it up with the dress in terms of style, because you'll just end up with something strange you'd never wear again. And there's no reason you can't use a patterned one, like red and white.