Drop shadow as light bleeding
How to use it
Apply a drop shadow to an element and set it to the same colour as that element. Give the shadow no offset, and increase its size until it's visible on all sides of the element.
When to use it
If you have an element that's meant to look like a light source - it's glowing - then a drop shadow like this can simulate that light bleeding onto the interface around it.
What to watch out for
I've used a dark background in the example below because with some colours, you won't get a very good effect unless the background is also the right colour to show it off.