The traditional way to add color to a black background is to do it in the studio, and not use a black backdrop at all. Instead, under expose a grey seamless background (turning it to black) and light it with a flash behind the model which has a colored gel over it. What about an image exposed with a black background? It turns out the change can be made fairly easily and quickly in post production. Here is the starting image of Dolly, which has been retouched and flattened in Photoshop:
The model first needs to be extracted from the black background. Please refer to my blog post "Rescuing Underexposed Images for Compositing" of April 8, 2014. The mask does not need to be precise. We now need a layer without the mask. With the layer thumbnail selected, Command (Control)-click on the layer mask to get a selection of the model called the "marching ants." Command (Control)-J to put the model up onto a new layer.
Next a variation of the technique described in my May 6, 2014 blog post "An Interesting Retouch for Studio Glamour - Part 4" will be used. Add a blank layer between these two layers, and fill it with black. The result should look like the starting image above. Add another blank layer above the black layer. I used the Eliptical Marquee Tool to draw an oval on this layer. I positioned it with the Move Tool (V) and filled it with color:
Remember to deselect Command (Control)-D before doing anything else. Next, run a blur. I used a Gaussian blur at its maximum setting of 1,000 pixels. The result seemed a little weak, so I simply duplicated the layer. Now select the top layer, the layer with the model but no layer mask. Try removing the black matte (Layer -> Matting -> Remove Black Matte) and see how it looks. Here is the cropped final result: