A couple of people have done project reports for this use case. It’s typically just a matter of having the bulbs turn pink or blue.
The following one was particularly cute, I think, and used two light strips on a white Christmas tree. There’s a video so you can see the full effect. You can use the same idea but just use two different light fixtures. I especially like the way the effect is presented in this project because the lights are blinked between the two colors for one minute before settling on the final result. I think that builds the suspense and also gets people’s attention focused on the reveal device before it happens, so they don’t miss it. . 
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I also like your idea of having an echo announcement, which wasn’t possible at the time that project was done. I might add a little music first, again just to let people focus on the announcement device before it happens.
And congratulations to whoever is expecting the baby!