cairo-set-source-rgba (cr red green blue alpha)

cr -- a cairo-t context
red -- a double float red component of the color
green -- a double float green component of the color
blue -- a double float blue component of the color
alpha -- a double float alpha component of the color

Sets the source pattern within cr to a translucent color. This color will then be used for any subsequent drawing operation until a new source pattern is set. The color and alpha components are floating point numbers in the range 0.0 to 1.0. If the values passed in are outside that range, they will be clamped.

The default source pattern is opaque black, that is, it is equivalent to

(cairo-set-source-rgba cr 0.0 0.0 0.0 1.0)  

The color arguments are converted to the type double-float before being passed to the C function. So you can enter any number as an argument for the colors.