Piet Mondriaan was a Dutch painter who developed a distinctive form of non-representational art.
Even if you don't know Mondriaan's name, you may recognise the style: white background and a black grid, partly filled with three colours. It's simple, but can also be very eye-catching.
Mondriaan Creator is a free program which automatically generates Mondriaan-style images.
Launch the application and it immediately displays a random image. Keeping clicking "Create painting" to generate something new each time.
By default most of the grid will be white, but you can tweak this colour balance to suit your needs: less white, more blue, double the yellow, whatever you like.
A "Shape Settings" dialog offers a choice of two grid types, as well as setting the minimum grid space to be coloured, the width of the grid lines, and more.
The painting resolution starts screen-sized, but can be increased up to 4800x4800.
When you're happy, click File > Save to export the finished image as a bitmap.