EXcalibrator requires red, green and blue images saved in the FITS format. So the first step is to split your color TIFF image into the three separate RGB FITS files. The color TIFF image must be unstretched and linear. The freeware program, FitsWork, does a nice job creating the three color FITS images. I suggest saving them as 32-bit floating point.

Next, one of the three FITS images must be plate solved. The free online service, Astrometry.net, works quite well. With a successful plate solve, the website creates the file "new-image.fits." Download this file to load into eXcalibrator's WCS text box.

Shown, above, is a typical eXcalibrator solution.

Load the TIFF color image into Photoshop and stretch it so that the background level comes up to about 30 on the scale of 0 to 255.

Put a copy of the image on a new layer and set the blend mode to color. This allows us to use the original image for the luminance and the Color Layer for color adjustments.  Finally, make sure that the color layer is selected.

Use the channel mixer to make the color adjustments as calculated by eXcalibrator.

Here is the result after applying the eXcalibrator adjustment factors.

With this example, it is not surprising that eXcalibrator introduced a blue-green color bias to the background. We can easily see this in the above color histogram.

The final step is to set the background to a neutral gray. Use the Levels Tool to lower the black point for the blue and green to match the red. This subtraction process has no visible effect on the color for the much brighter stars and nebula. In this example, the green was lowered by four and the blue by three.

Finally, flatten the image and were done.