Our ceilings are light blue, but we haven’t mounted anything but the (white) can lights yet, so I doubt my pictures in the building a house thread would be any help. However, I’ve seen it done (e.g., the house I grew up in had light aqua ceilings), and it’s fine. It might be jarring if the ceilings were dark, but as long as it’s a pastel-adjacent color, you should be fine.

Have them sprayed the same color as the ceiling.

We do this with all ceiling colors including colors of white.

When you see pictures online there actually may be something in the ceiling but it's been photoshopped out. So this is probably done more than you think it is. As Lori says, paint whatever it is the same color and go for it.

While vents can be spray painted, detectors cannot be sprayed, correct??

How many detectors do yo have? Can you move it to a wall?

Do not paint smoke detectors ever. AC vents no problem . No smoke detectors work best on the ceiling that is why they are mounted there . I do not see the pic you are referring to but honestly I rarely notice people's ceilings

The vents can be painted to match the ceiling.

Smoke detectors should not be painted, but we are so accustomed to seeing them in our homes and spaces that we simply stop noticing them.

You will notice it when you first paint because you will be hyper aware of the issue. Within weeks you will stop noticing. No one other than you will notice at all unless you point it out.

I painted mine to match the paint colour.