Can you set a wallpaper on your 3ds? 

Also I don't know which colour to get   Think I might go for cosmos black, I'm also putting it off because everyone keeps saying Nintendo will bring out a new model with a second circle pad, heroes of ruin has got me really interested along with resident evil revelations, so Many great looking games!

No, you cannot set a wallpaper at this point. That isn't to say such a feature won't come in a system firmware update, but there's really no way to tell.

Well, I got a Aqua Blue 3DS... simply because everyone seemed to be getting the Cosmo Black ones. So I figure somewhere down the line the Aqua Blue systems will be more valuable. Also, Heroes of Ruin does look amazing indeed, there's tons of great games on the horizon. And now that the sales of the 3DS have exploded as of late, we can only expect tons more amazing games to be announced in the next year alone. Luigi's Mansion 2, Tekken 3D, Kid Icarus, Beyond the Labyrinth... not to mention the great games that are already out. I received Super Mario 3D Land for Christmas, and let me tell you... there's a great game right there. I also have Zelda: Ocarina of Time 3D, Dead or Alive: Dimensions and Ridge Racer 3D, all of which are a blast.

If you do get a 3DS, be sure message me on here and send me your friend code. ;)

I will thanks, I'm trying to decide between flame red and cosmos black and still keep changing my mind lol, yeah there is so many games I want mario kart to, Nintendo and 3ds I's all kinds of awesome right now and long may it continue :)

Black, or the black with Zelda logos (sexah!)

can't change the "wallpaper" but you can change the layout of the channels from sidescrolling across one line of icons or have them laid out in multiple grids