Colour-cube Outline is a Hexagon (b):

The three-dimensional representation of the HSV model is derived from the RGB cube. If we imagine viewing the cube along with the diagonal from the white vertex to the origin (black), we see an outline of the cube that has the hexagon shape illustrated in Figure. The boundary of the hexagon represents the various hues, and it is utilized as the top of the HSV hexcone .In the hexcone, saturation is measured along a horizontal axis, and value is along with a vertical axis through the centre of the hexcone.

Hue is represented as an angle around the vertical axis, ranging from 0° at red through 360°. Vertices of the hexagon are separated through 60° intervals. Yellow is at 60°, green at 120°, and cyan opposite red at H = 180°. Complementary colours are 180° apart.