Creating a Solid Colour Background in Photoshop

Start by opening your image in Photoshop.

Select the background – you can use whichever selection tool you are most comfortable with but it is important to have a look at your image and see which tool would work best with the image:
a.            Lasso tool – this is a freehand selection tool and will need quite a bit of patience accuracy to use
b.            Polygonal Lasso Tool – a bit easier to use as you select the area ‘point-by-point’ and can make a more accurate selection
c.             Magnetic Lasso Tool – an ‘automatic’ version of the Polygonal Lasso Tool – you run your mouse over the outline of your subject and the area is selected – although you will have to tidy up a bit at the end of the process
d.            Quick Selection Tool – Similar to the Magnetic Lasso Tool, but with less accuracy
e.            Magic Wand Tool – Selects area by colour – best used when the area you would like to select is one colour/tone

Once the area is selected – you can decide how to go about changing the colour, if you would like it to be darker/black I would suggest you adjust the brightness, contrast, highlights and shadows to achieve this – it results in a more ‘realistic’ image. The background area above the dragonfly was selected and teh contrast and brightness adjusted - the image is much darker but has retained some hilights and shadows.

A straight ‘colour fill’ often looks as though the image has been cut out and pasted on a background and does need some tidy up after the background has been adjusted. The background on the bottom half of this image has been colour filled with black - the colour is very 'flat'

When Tidying up, you can use the history brush to ‘recover’ some of the bits that have been covered over (Note that if you would like to use the history brush on any image, never crop until you have used it – the history brush relies on ‘memory’ so if you crop the image, the brush does not recognize what you want it to)
If you have a solid colour and some of your edges are a bit harsh, select the Brush Tool, select a round/soft edged paint brush, zoom in and carefully smooth out the edges.  You can also blur the edges using the Bur Tool – again you will have to select a suitable size for the tool
It’s a good idea to play around after you have changed the background – see if the image needs a saturation adjustment, etc.  Experiment a bit and have fun!