If your book is not a textbook or technical reference, then there is no substitute for graphic design. The design does not have to be exotic. It merely needs to convey the impression that "this is not a textbook or technical reference" to potential readers.
If you will be using a print-on-demand service, nowadays most of them are happy to accept a PNG or JPG image, prepared according to their standards. Their web sites will explain. You may also be able to provide a PDF, but in most cases that would only be a wrapper around a single graphic image, not a file produced by TeX.
Unless it is a solid color, the background should be created in a raster graphics program. Usually at 300 pixels per inch. You can add text there,
or create text in TeX, then import the result as layers to the graphic. If you use TeX, you do not need to worry about positioning or rotation or color,
since you can do that to the imported text in the graphics program.
The finished cover should be flattened (no layers), which rasterizes the text. It will always be rasterized before print, so you lose nothing.
Depending on the print service, you may be required to submit RGB, or CMYK, or your choice. You may be required to attach a color profile, or omit a color profile, or your choice. Given the choice, it is best to submit as RGB
without attached color profile. Let them do the color conversion.
Be sure that the text does not have any excessively fine structure. The four-color printing process tends to make fine (small) text hard to read.
The printed color will be in a different color space than you saw on screen. Without a calibrated monitor and graphics art experience, just live with the effect of color conversion. In general, very brightly-colored reds, greens, and blues will print less brightly. Dark colors (deep browns) will appear slightly washed-out. Black may not be as black as you expected.
Professionally printed books use a different print technology. That is why those "instant best-seller" books can have brilliant reds and deep blacks. Same with textbooks, which can be printed using expensive technology, because students have little choice.
You are strongly discouraged from submitting vector artwork. Some services refuse to accept it. If you wish to design using a vector program, then import to a raster program, and finish it there.