For this project, I used the app PhotoScape X, and there is a free version and a Pro version at 35 USD (one-time cost). This app does not sponsor me,
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.
I created the next one with a bright yellow background, then added the triangles shapes in neon blue and purple. The last step is an already created swirling shape in the app, and I just changed the color of the design.
So go crazy, add circles, squares, and triangles in different sizes and colors, use paintbrushes to create blobs of colors, or add premade filters to a background and change the colors.
Some other examples of abstract backgrounds.