(color-rgbaf r g b a)

color-rgbf returns a color made from four components (rgb and transparency)

Parameter	Description
r	Red channel value (0 is black, 1 is maximum red)
g	Green channel value (0 is black, 1 is maximum green)
b	Blue channel value (0 is black, 1 is maximum blue)
a	Alpha channel value (0 is most transparent, 1 is completely solid)

Example

Example 1: Return the color Papayawhip (255, 239, 213) with 25% transparency

> (define Papayawhip (color-rgbaf 1.0 0.937 0.835 0.75))
> (color-alpha Papayawhip)
191