Open a new document with a white background.

Fill diagonally (top left to bottom right) with a black to white gradient (you can experiment with different gradients and colors later).

Filter, render, difference clouds.

Command I (inverse)

Command L (or enhance, adjust lighting, levels) and adjust until it looks like lightening.

Command U (or enhance, adjust color, adjust hue/saturation). Adjust hue until you like it.