I genuinely like non-white pocket squares. But in this case, I'd recommend a white square.
Maybe a cotton or linen one, not silk, with a colored border. A navy border would be the obvious choice with that tie, although depending on what else you're wearing, some other color might work well, too.
See, a lot depends on what color suit you're going to be wearing (and you neglected to say), as well as what color shirt (and you also neglected to mention that).
The Tie Bar offers a selection of white cotton pocket squares, with colored edges, for about $8 each. At that price, I figured I'd pick up a few. They're not the greatest pocket squares in the world, but they're okay, and for 8 bucks I can't regret buying them.
Of course, since you're asking for help all of 72 hours or so before the christening, you haven't time to buy by mail. You're limited to those pocket squares which you already own (and you haven't told us what they are), or pocket squares available from retail stores near you (and you haven't indicated where you are).
But in any case, congratulations on becoming a godfather.