STEP 1
Open the picture you want to add the effect to. Make sure your layers palette is open (Window- Show Layers)

STEP 2
Make a new layer (shift + control + n). Click on the paint bucket and fill the new layer with color #0000ff.
STEP 3
Change the blending mode to Exclusion and lower the layer's opacity to around 30-40%. You're done! This effect kind of gives pictures an aged vintage look.
