code: def mix(self, ca, cb, xb):
          
          r = (1 - xb) * ca.red + xb * cb.red
          g = (1 - xb) * ca.green + xb * cb.green
          b = (1 - xb) * ca.blue + xb * cb.blue
          a = (1 - xb) * ca.alpha + xb * cb.alpha
          return gdk.RGBA(red=r, green=g, blue=b, alpha=a)
docs: Mix colors.
  
          Args:
              ca (gdk.RGBA): first color
              cb (gdk.RGBA): second color
              xb (float): between 0.0 and 1.0
  
          Return:
              gdk.RGBA: linear interpolation between ca and cb,
                        0 or 1 return the unaltered 1st or 2nd color respectively,
                        as in CSS.