Making Backgrounds in Illustrator

 This is a monochrome I created. There is a solid base layer. The fan shape is a blend of an irregular brush stroke. The shape in the opposite corner was created with the pen tool and a gradient.

 This was created with the arc tool and the blend tool. Then I duplicated and reflected it and changed the colors. The bottom layer is a gradient blend.

 Here I used text to create the background. I just played around with different fonts, sizes, colors, and opacity. I decided to have some fun with. It's hard to work with words without giving them meaning.

 This has the orange base layer, then the green layer has the halftone effect applied with a gradient transparency mask.

This has a red base layer with a halftone gradient transparency mask. The horizontal bars were made with the blend tool and a gradient. The circles were made with the shape tool and an irregular brush. They have a gradient transparency mask as well.