These three different curves show three continuous color scales as they interpolate between two colors linearly, except the interpolation happens in three different color spaces: RGB, Lab, and HCL. The points in the curve are plotted in Lab coordinates, where increasing luminance points “up”, and the ab coordinates range in the other two directions: horizontally and into the screen. The wireframe cube delimits the Lab space in 0 <= L < 100, -100 < a < 100, -100 < b < 100. The circle delimits the values with Chroma less than 100.
Can you tell which curve uses which color space?