I'm looking for suggestions for a portrait background (paper/cloth). I have seen photographers using different background screens, but being an amateur photographer I don't want to invest money buying many backgrounds - instead I would like to know which would be the best to use for many purpose. That being said I should be able to change / edit background color in Photoshop after the photo shoot.
If you want just one background I'm going to suggest white seamless paper because it's the most versatile, some examples:
Point a flash at the background directly behind you subject and it becomes pure white (and easy to mask in Photoshop)
Put a soft light source very close the the subject and the background becomes black (or play with the subject-light distance to get different levels of gray).
Use a softbox at an angle to create different light-dark gradients on the background.
Point a flash with a colored gel at the background to make it any color you want.
The size depends on what you are photographing, for head-shots you need a pretty small piece, for full-length group shots you'll need a huge backdrop - look at a typical portrait you like to make and just measure the width of the backdrop to cover your photo's background.
And about where to buy - any good local or on-line shop should do.
I'll follow your example and then give a bit of an explanation...
Color : Green or Blue
Material : Paper
Screen Size : 6' paper is fine for head shots, you can go longer
Where to buy: Any good photography store
What you want is the ability to key out the background and that is what green or blue paper gives you. It's fairly low cost and not all that hard to do. Big things to remember if you want to knock out the background is:
Avoid shadows on the paper.
Keep the subject far enough away so as to avoid having the background reflect on them.
Make sure your subject isn't wearing the same color. A little easier with two colors to choose from, though you're toast if they wear both... :D
I suggest paper because it's relatively low cost, you can discard the dirty portions, and it's easier to keep smooth and even.
The term you want to search for here and at Google (or your search engine of choice) is "chroma key."
We use sheets for some of our backdrops, just be careful of wrinkles and don't let your subject stand too close. Keep an eye on clearance sections at Walmart or target and you can find queen flat sheets for $6 to $10. I bought light gray, white and flat black and use them often.