Although computer colors are made by combining
red, green, and blue (
it can be more intuitive to think of a color in terms
of its hue, saturation, and lightness.
c = hsl(270, .8, .5)
The program above specifies a purplish color. The "Hue" is a number that ranges from 0 to 360, representing a color on a color wheel (pictured below). "Saturation" is a number from 0 to 1 giving the intensitity of the color, and "Lightness" is a number from 0 to 1 which adjusts the color from blackness to whiteness.
hsla function can be used to create a partially
transparent color code. The fourth component "a", or "alpha"
value is an opacity level from 0.0 to 1.0 that determines how the
color is mixed with the colors "underneath".
So, for example, if a dot is drawn with alpha 0.5, it is blended with the colors behind it:
bk 100 dot red, 200 fd 200 dot blue, 200 bk 100 dot( hsla(135, 0.8, 0.5, 0.5), 200)