With the Color Curves tool, You can change the interpolation type to smooth mask or to select semi-transparent area.
Original.jpg 19.57KB 286 downloads Smooth.jpg 19.8KB 298 downloads Blend.jpg 24.69KB 297 downloadsg]
You can also use the Curves to select areas by hue or by saturation for color correction.
Red.JPG 31.59KB 300 downloads Blue.JPG 35.25KB 278 downloads
Curves can even control gradient colors to make it not linear!
Any kind of interpolation (cubic, step, repeating ping-pong) is available.