Just published.
http://gimpscripts.com/2016/12/whitebalancestretch2/
Starting from the initial version I added a procedure to remove the remaining colour-cast.
For those who have photos suffering the colour-cast problem.
It has no prerequisites.
It works at single pixel level in the added procedure, so -even if it makes use of Tiles- it could be slow for images larger than 1600*900, but running time is still acceptable IMO.
As a sample of how it works I took an image posted here on Gimpchat:

applying this new filter I got this outcome


Hope some photographers can test it and evaluate.

Looks good. Might try it next week.

Thank you Dinasset. I think that it's going to be a fantastic tool in photography. I did a first checking and it's works wonderful.

and these.

Dinasset, this looks like an advanced automatic white balance. Good. I will study your script . I hope I can understand what is happening.

With "enhance outcome" on, it removes all original layers. I think, that can't be your intention.

iarga,
you're right,"lapsus calami" (used flatten instead of merge_down)
try this amended version

I will try this on some old faded slides that have lost much color information.
Thank you!

Dinasset. It's ok now.

For an automatic stretch/ color balance, this plugin does a very nice job.

For green under water pictures, it is very good, better then others. With blue/cyan underwater pictures it depends on the image. But other automatic white/color balance plugins have also difficulties with this.

Maybe an idea for another plugin, is to add the possibility to choose (a) "neutral point(s)" (in shadows, midtones and highlights). Another addition could be to have "buttons" for daylight-tungsten, tungsten-daylight.

But that plugin isn't fully automatic anymore of course.

Your plugins are getting better and better.

Thanks, iarga, for your nice comments.
Still not perfect (on the example image a blue nuance was left...).
Could be an idea to ask for a White/gray/black point (as an option).

But please make those "gray" points really neutral. I mean, use those points only for color balance, not for white balance. That's the reason I wrote "gray" point. But maybe "neutral" points is more clear. Sorry for confusion, I did edit my post.

I didn't ask for black points and white points (as in GIMP levels). That's something different . But of course that can also be an addition.

OK, my "white/gray/black point" was just to say one neutral point (not 3 points).
Anyhow I will think about, because my personal experience with such kind of "input" was almost always bad (also using other software).

Thank you Dinasset. I do understand you. I have "tested" a lot of this kind of plugins/ software.

I agree with you. This filter now has the tendency to overdo the colorcast compensation
However, I like your idea to use histogram values, so I hope you will continue this work:

iarga, would you like to try this version? tell me the general effect, not only for Lena, but also for those images you tested before

Dinasset, I see a little improvement (highlights). But the difference is very small.

edit: Downturn: but you also lose some improvement in the shadows.

And with this one?

Again little improvement in highlights, but also a little bit worse in shadows and midtones also.

OK, I stop now.
Tomorrow I will think about.
Thanks you for your support.
Good night!

I will support you, good night also. I'm busy tomorrow, so I will have a look tomorrow evening.

Wow, I'm delighted, thank you, this little number is over 40yrs old.