Here are some gradients and a tileset for your levels.
The Simple Gradients are a set of primary, secondary, tertiary, and quaternary hues blending into black. Do not credit me for those, they are too easy to make.
The Simple Color Blocking Tileset is a neon color block tileset of simple colors. Please credit me for the tileset, but not the gradients (you can use the simple gradients any way you please).
Edit: The Simple Gradients now include four shades of grey (including white)!