Decisions about color are always going to require pause for reflection. You may choose colors because of the memories they evoke, for cultural reasons, or because they are soothing. There is no doubt that the best way to do this is to practice.
These choices are all personal, and only you can make them. Some designs can be helped onto wall or floor with stencils and stamps. Some design classic will always remain, but they can be adapted to suit the current look. The wall you are to work on should be well prepared and white.
These are an excellent way of controlling where your paint goes, as well as a means of covering a surface quickly. Whatever you do, always look at other interiors, at textiles, clothing, and painting, and take your lead from those you like.
Blue with green, for example, is perfectly acceptable. You may seek guidance in color theory, but understanding the spectrum and how to mix paint will not be much help in choosing which colors to use, although it can explain why certain color combination behave in the way they do.
These are simply ways of applying and manipulating paint to create a range of surface texture. The end result will always have a quality that cannot be matched by any wallpaper.