A couple of things jumped out at me.
1. I'm not positive but I don't think patio paints are meant to be used in a submerged environment. Rain OK, occasional soaking OK, underwater? Probably not OK.
They are called "patio" paints and patios are seldom submerged.
2. Not to disparage Rust-oleum sealer but there are better sealers out there. And again, is the product OK to be used underwater? It says ideal over concrete, etc. but does it say "can be submerged"? Also a lot of sealers are meant only for vertical surfaces, they're basically water "shedders" instead of waterproofers. Read the label carefully. I use a Behr brand sealer that is meant to be used around pools.
Note: most sealers go on cloudy but cure/dry clear.
3. Re: painting a birdbath. Birds have very sharp talons, any little scratches through the sealed surface will allow water underneath the sealant. This in turn will either react with the concrete causing it to flake or become grainy or let "bubbles" form under the paint and sealer skin.
4. Was your concrete leaf completely cured? If not, any paint/stain/sealer applied to it may come off.
5.I wouldn't seal and then paint, the paint may not adhere or go on smoothly.
6. Personally I would recommend using a non-painted leaf for the bath and using a painted leaf as decor. Perhaps paint the under side of the bath leaf?
Don't know if any of this helps or not. Let us know how things turn out.

Check the hypertufa forum.....
There are a couple of posts there that talk about leaves.

I've had some sleep since my last post so my brain...is in a higher gear let's say.
The best way I've found to color concrete/hypertufa is with wood stain. If you use a verdigris stain you'll get a green-yellow color, maple stain will give you a reddish tone. The stain soaks into the concrete instead of sitting on the surface like paint. Follow up with a good sealer and you should be in business for quite a while.
Of course you should experiment a little first but I think you'll like the results.
Good luck!

You could also use concrete colorant when mixing up your concrete. It comes in a lot of different colors and then when you apply your sealer, the colors really come up. I prefer that to trying to paint concrete with any kind of paint. Sealer needs to be reapplied periodically as well because it develops tiny cracks and holes that can allow moisture into the concrete and will destroy your project eventually. Cheryl

Well that all makes sense to me. I have colorants but love the look when I can blend my own colors. I made the leaf look like a fall colored leaf. I guess just don't use it as a functioning birdbath!

I've been researching this a little and found some info on another forum. This is from someone who paints concrete statues for a living. She seals the item with behrs sealant first, then paints two coats and then seals it again. She uses an exterior latex paint or outdoor craft paint.

I'm going to use acrylic craft paint and see how it holds up because I can't afford to buy anything else for the small quantity of each color that I would use.