I think if the color i son all four walls and the ceiling one shade lighter may be a better choice. If you google Tame teal, you'll see a few rooms in that color.
Now, the other option is to paint the ceiling in Tame Teal and then reevaluate: Finish all walls in TT or switch to Aquaduct.

I had a BR like this in my parents' house. We had wallpaper on the "angles" and then my ceiling was white. Seriously, I do not remember what the non-angled walls looked like. I think they were painted vs. wallpaper. Hey, it was the 70's LOL.

My oldest niece's BR was also the same. They painted everything a light aqua.

Here's a fairly dark color with a ceiling painted the same. I think it looks terrific and does kind of make the ceiling disappear, which makes it seem taller.

[Traditional Wine Cellar[(https://www.houzz.com/photos/traditional-wine-cellar-ideas-phbr1-bp~t_755~s_2107) by Charlotte Interior Designers & Decorators traci zeller designs

I think all the same. Painting the angled part lighter will cause a long horizontal stripe effect.

I can't promise that it won't be intense, you may want to go up a strip for the whole thing. I tenant picked this out in a house I rent out for her bedroom:

Yes, I was going to say what Pal said. I was just upstairs today doing touch ups in my son's attic bedroom, which is a lighter blue, but all walls and ceiling are the same. A long time ago, the ceiling used to be white, and my eyes went right to that white "stripe" every time I went into the room.

We have an aqua attic bedroom. It's a custom color-- as in I mixed a bunch of leftovers from various cans of Sherwin Williams paint and added some universal pigment so I will never be able to touch it up. It's all the same color, but lighter than what you are proposing and there is great western light in there. Guests fight over who gets the "honeymoon suite." LOL. I would still use all the same color if the natural light were dim, but I'd be sure to arrange for good artificial light, perhaps even some wall washer sconces, good reading lights, and adequate closet lighting, plus whatever else is needed. If the walls are in great, smooth condition, lacquer would be great!

Just moved our 13 year old dd into our attic last summer. We used BM's horizon and painted out the ceiling. I was a bit worried about doing it all one colour, but it is sooooo much better. Not as intense a colour as your looking at, but it is not white by a long shot.

Click on this link and it will give you an idea of how it would look.

Here is a link that might be useful: Aqua Attic

I think it would look good with the ceiling the same color. And I don't think you should lighten your daughter's choice, it's her room and it can be painted over in a few years. I remember when I chose a beautiful deep gray-blue for my bedroom and my mom thought it would be too dark (I had ivory furniture and I wanted the contrast) so she got the paint many shades lighter. I never liked it as much as the color I chose, and always wished she'd let me choose for real! Kids need the ability to make real choices in their lives, I think.

I'm in the process of converting my recently-moved-out son's room into a guest room, so I took before pics and will take after pics when it's done, but this "in process" pic shows the ceiling and walls painted all one color. Sorry, not the best quality photo.

Okay, you've convinced me to go with all over color and the shade she picked out. Personally, I think a lighter color would be more livable, but I have my own bedroom to live in.

My dd went away to camp and just told me to pick a nice colour while she was away. Much easier. Luckily, she loved my choice.

If it's an intense color I would paint the ceiling the lightest color on the strip. I know it's trendy to paint the ceiling the same color as the walls but with dark or intense colors I think it's often too much.

I chickened out on the painting of the attic master we built. Painter was there working and I panicked. I thought I wanted warm and cozy and chose a tobaccoey brown-gold. As soon as the painter got going on the angled walls, I was horrified at what it did to the color--reminded me of the yucky yellow of a newborn's first messy diaper, and bright! The vertical walls were fine.

So I made an abrupt swivel to a lovely, dark-medium taupe. Once again when he got going the color on the angled walls was distinctly different from the other walls--this time so much darker than the vertical that it was already starting to look like a cave in there (windows on one vertical end only). It was broad daylight and it just didn't occur to me to have him sit down while I brought a lamp in to try it out. So I defaulted to the lightest taupe on that same card, and now it looks like a subdivision developer chose the color. Whatever.

In my space, the ceiling strip, in natural light, was lighter than the angled walls with all three colors. For me those, and how colors acted on them, were the issue to deal with.

I think it will be fun and cozy. And, like you said, it's only paint.

DD2's MBR is painted a pretty blue/green color - special mix working with the best man at our local paint store. She doesn't have the wall color in many/any accessories in the MBR, but did carry it into the MBath a bit. You can see a little peak of her bedroom in the bathroom reveal, linked below - before picture with dog standing in the doorway. I have yet to take pictures of her master, but plan to soon. She uses orange, yellow, dark pink and brown as accent colors.

Here is a link that might be useful: DD2's MBath

Do you have any white paint lying around at home? Because what I'd do is paint the walls the aqua and paint the strip of ceiling white.

Then if I didn't like that, for whatever reason, I'd take some of the leftover aqua paint and mix it with a bit of white, to tone it down a bit, and paint the ceiling with that.

Then if I didn't like that look, I'd paint the ceiling full-strength aqua.

Growing up in an 1880s Victorian, we all looked forward to moving up to the attic rooms when we were teens. All the rooms had sloping walls, with varying amounts of ceiling. My parents let us go a bit crazy with the paint.

Just based on my experience, a tiny bit of white ceiling can look a bit odd if the wall color is dark. If the wall color is fairly light, the white ceiling doesn't stand out as much.

If you want more than one color on the walls, you need to make the two colors very different colors, not shades of the same color. For example, one room was seafoam green on the slanted bits and dark faded red on the vertical bits and it actually looked pretty good. And there was the lime green and dark blue room, which again, looked better than you'd think. The darker color looks best on the vertical, with the lighter color on the sloped portions.

Your daughter's room won't seem like an aqua cave if the floor, bedding, curtains and other accessories are different colors. Get some fun colors on the bed and floor and see what happens.