Well wallpaper can be a pain also if you ever want to take it down.
Now I've seen people paint over it with good results, that is if it's not wrinkled or textured to begin with.
You can paint walls any color you want as long as you keep in mind you may want to redo it someday and plan for it. I still remember the time I was going to do my boys room in a Dallas Cowboys theme their favorite at the time, after the bed spreads and curtains where ordered and paint bought, they decided they liked Green Bay.
Think the first I heard about that was when the youngest said something about "stinking Dallas Cowboys". Just remember plans change.
Spend time in a paint store (not home depot or Lowe's) and ask questions. You can buy where ever you want but the people working at those places rarely know much about the product they sell (there are always exceptions I suppose).
Oh, and don't waste your money on latex thinner, you'd be surprised how many times I've seen people do just that
