This is one of the most awesome images I've seen at APOD

http://apod.nasa.gov/apod/ap150113.html

something exploded so symmetrically!

I love NASA photos.
Something to explore,
openFITS - Create 3-Color Chandra Images from Raw Data

I've attached my Chandra results still in layers as an xcf file. The layer modes are still screen.

I can't make the different layers in different colors and have them all show up as different colors in the top image. I used Testing/Photocomix/Smooth for the smoothing.

Did you rename and order them accordingly?
Cas A 4.0-6.0 keV (hard) Fits File [4.0 MB]
Cas A 1.5-3.0 keV (medium) Fits File [4.0 MB]
Cas A 0.5-1.5 keV (soft) Fits File [4.0 MB]

My Colorized settings
Hue: 240, 120, 0 (top down)
Saturation: 100
Lightness: -50

Mine is not identical to the Harvard image but shows distinct color.
As you can see from the noise, I was quite aggressive with gamma Levels.

Yes. Download my xcf file and you can see that I did.

I started with the bottom, colored it red, used the recommended sat and lightness.

And went on from there.

The one I attached in my last post was done from scratch.
I had downloaded yours but didn't notice the colourization so I just started with the original 3 .fits and did a new one.

Opened yours. Still playing with mine. What in G'MIC did you use? Only anistropic smoothing one I could find was Testing / Ronounours / Smooth

Repair / Smooth [anisotropic]