Add noise and textures
I make a new layer and fill it with 50 per cent grey, then add 10-15 per cent Noise. I set the layer to Overlay and adjust the Opacity slider to 30 per cent. Gaussian Blur softens the noise. The watercolour texture is made by painting black watercolour on to wet paper. When dry, I scan the texture into Photoshop, and drag it to a new layer with Layer Mode set to Multiply.