For this project I used the app PhotoScape X, there is free version and a Pro version at 35 USD (one time cost). I am not sponsored by this app,
I used texture filters, pre-created shapes, and bright colors to create digital Images. The one below is rectangles lined against each other with an added paper texture filter to create an abstract watermelon. I started with the dark green colors of the rind which merged into the bright red of the fruit, last stripe of color is black for the watermelon seeds.
The next one I created with a bright yellow background, then added the triangles shapes in neon blue and purple. The last step is an already create swirling shape in the app, I just changed the color of the design.
So go crazy add circles, squares, and triangles in different size and colors, use paint brushes to create blobs of colors, or add premade filters to a brackground and change the colors.
Some other examples of abstract backgrounds.