Since version 3.24, Gnome has a built-in blue light filter called Night Mode (AKA Night Light). It is now almost a proven fact that exposure to blue light in night-time disrupts our sleep pattern. Night Mode is there for that reason: to limit your exposure to both bright and blue light from the screen. It works by applying a red hue upon screen. So when it is turned on, blue color may look purple (this is the case here, see comments on the question).
Go to Settings > Devices > Display. There you can customize Night Mode by setting activation time or controlling red-level. Or you can disable the feature altogether.
You can also change it through commmand line.