Parodia ottonis has been a reliable bloomer for us over the years. Hardy yellow flowers on a small globular base.
Found throughout South America, it is generally solitary when young and when it starts to branch it can become a nice clump. The purplish color comes in winter and can stick around through the summer if given enough sun. If you prefer the green color you can give it some afternoon shade and that should green it up nicely.
These used to be called Notocactus but the DNA research is in and…. Parodia! Anderson supports the Parodia designation, but many cactus nurseries are still using Notocactus, so I guess the final jury is still out on this terrible question.