How do I achieve this effect in Photoshop?

Have just the subject on its own layer.

Right click the image of the layer in the layers tab and click "select pixels".

Fill in the selection on its own layer with whatever color you want and put that layer behind the subject layer.

Got to Select>deselect.

add gaussian blur the layer with the color on it.

Voila.

>>311614

Or, ya know, just add an outer glow layer effect.

>>311634
This

>>311609
outer glow or just use a drop shadow and change the color to a soft minella