Hehe, I'm no interior decorator my any means, but I'll give it a shot.
*whispers*
Just don't tell anyone that my room's a hodgepodge of hand-me-down furniture with everything from stuff from my great-uncle to my mom's college days. xDHehe, since its too hot here to sleep, I'll sketch up a few ideas in Photoshop real quick. I swear I'm getting this air conditioning fixed even if I have to crawl up in the attic myself. D: Its like 85 degrees in my room right now.
Anywho...
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Mmmm. I might stay away from black if I were you, but then again, its your choice. =3 Black usually weighs down a room, unless you're going with something like a black and white theme. White's almost the only color that can help balance out black.
Stencils are your friends. x3 You can find some really cute flower design stencils at craft stores, and even major retailers like Wal-Mart and Target. They're cheap, really easy to use, and can add a really nice touch when doing a paint job. Its easy to just paint all the walls one shade, then go back in with stencils and add a variate pattern to the wall.
Either that, or you can buy that paper sticky trim stuff with patterns on it. I'm certain they have rose prints, since its a fairly common theme for a room. I myself have had bad experiences with the stuff, but that's mainly because I find it a pain to stick on.
You might have better luck than I did, however. xD
There are a bunch of different routes you can take, with your rose design. I'll just throw out a few ideas.
=oThis is kinda what first came to mind when I read your description.
Its kind of a general design rule I follow to break up the meeting edges of the planes of the room. What I mean is putting something that takes the edge off of where the wall meets the floor, and the wall meets the ceiling. You'll see this in kitchen designs a lot, where they have the tile or backsplash to break up where the countertop meets the wall.
This is fairly easily done in a room by adding a thin strip of another color/trim. It also makes a nice canvas for other little designs if you're using stencils. =3 If you're using brighter colors, it probably would be a good idea to isolate them to the trim areas, otherwise the room will look too busy and confusing for the eye.
=DFor this one, it'll take longer to paint, but its still relatively simple once broken down. Walls painted the major shade first, with the stripes and trim in the middle added after the first color dries.
And if you want some cheap decorating tips, hanging art on the wall is always a nice touch. For your rose room, you can simply buy some silk roses from Wal-Mart, cut the blooms off, and stick them behind a glass frame with a matte board background. Its quite cheap, but still looks nice. x3
For paint colors, I'd most likely stick to softer, muted colors with more white in them. Brighter colors can be used for smaller designs and accents on the walls, but personally I don't find them too appealing when they're the major color on the walls.
They're also a pain to try do decorate around, since you'd pretty much have to match the colors on your furniture in order to make it work.
Muted colors work well, because you can use white furniture and still make it look nice.