I have this pattern (not started yet) & pattern instruction gives various colours for different parts then says "all other backstitching in DMC 310 (black) so I'm assuming since prayer is not specified that it is black. However you can choose whatever colour you'd like.

If you use DMC #3371 it is a little softer to look at than black thread.

Thanks to maubennet for the suggestion. I'm going to use #3371 as you suggested.