Inject colour effects into your day or night shots
Create a new layer and fill it black so you can see what you’re creating. Click on the Gradient button and choose a bright colour to transparent style, which will emanate from a corner. Do this again to build up colour, with the mode set to Screen.
You may wish to add even more colour. Simply select a big, soft brush and set this blend mode to Screen too. Lower the Opacity of the brush, so it’s not too garish. Then colour over the light leak in the colours of your choice.
Set this layer’s blend mode to Screen. The black in the picture will become invisible for you to only see the light leak, but by creating it on black, you can get an idea for what looks good! Experiment and study other light leaks for inspiration.
1. Scatter brush
Go to Brush Settings and select the brush. Set the Scatter to 30% and the Spacing to 100%. Then, when you brush, the dots will automatically scatter across the picture, like spots of dispersed light, ready for you to throw bright colours over later.
2. Layers and blurs
Scatter dots over the picture on a new layer. Then create another new layer and do the same – do this a few times. Then head to Filter>Blur>Gaussian Blur and blur the dots by 10px. Give each layer a different level of blur ready for the colour.
3. The finished piece
Now for what will make the picture: the colour. Create a new layer, below the Bokeh layers but above the Background. Place a coloured gradient, and set each of the layers to the Overlay blend mode. Merge all of the layers together and set to Screen blend mode.
You may find more good tutorials on : http://blog.photoshopcreative.co.uk/blog/tutorials/create-bokeh-and-light-leaks/