First Try

[Sam Smith]

This my first at replacing the background color. Please C&C.

[carregwen]

What was the background before?

I always find it best to shoot against the colour you actually want as it looks more natural. You can even do it outdoors. I took THIS shot of a Primula Viali outdoors using the black side of a fold-out reflector.

I think yours is fairly obvious that it has been changed. It's always better to try and achieve basic things like this at the time of shooting. If you are indoors, try using a large black art-board from an art shop. I use them all the time, and they are very cheap (few dollars)

[Clactonian]

Rob is absolutely right and it is always useful to have some card of various colours in your home kit. Sugar paper can be rolled so is more transportable for location work and is extremely cheap.
You can still get away with changing the background colour but you may want to try a feather edge on you selection or cut out to lose the harsh edge transition.

[Sam Smith]

Thanks for the feedback. (Rob read your post on people posting more) I will have to try to plan better. Here is the shot before I did anything other than a crop.