Now we're cooking with charcoal.
I've been learning about color recently. As I understand, color has three dimensions: hue, saturation, and brightness. To make the arrangement seen above, I started with the pure blue at the top middle: a specific hue, completely saturated and bright.
To the left, the saturation decreases. The leftmost column is the least saturated or the most 'washed out'. To the right hue (measured in frequency) increases, resulting in purple at the far right. From top to bottom, brightness decreases. Different brightnesses produce different 'shades' of color.
In short, hue determines the color of the color, saturation determines the intensity of the color, and brightness determines the shade of the color.