I disagree with the "simply"

I doo it octane and there's no other it then restaring the render, and by restart I don't mean reload... just on-the fly changing...

Actually this should be possible at Exporter stage, having a rough Preview in your viewport.
I totally disagree about your issue having to tweak colors to take long!!!1!
The idea in generating 3D content is mostly about knowing what you like to archive BEFORE hitting the render button (super important neo0.!!!).
You basicly first finish the 3D content, then setup materials & light, and then testrender...
By doing testrendering at the end of this workflow you can simply have a preview image as a base to modify ALL the materials & stuff.
Trial & error should be done after the third testrender...

True, but but there is a noticeable difference between the colors in your exporter viewport and those in your indigo render. Two reasons :
1) I highly doubt your exporter viewport is powered by an unbiased renderer. The colors in indigo are much more "accurate" than any exporter..

2) Using lights sources can make certain parts of your render lighter or darker or even change their color. This depends on your light settings and it almost impossible to predict... For example, if you have a blue scene and use a yellow emitter of 50 watts, what will be the final final color in your render? Impossible to predict.

In maxwell im guess that you can do more than just change the color.. You could change a glass material to a phong for example..

Afaik you can't...
You can change from phong to diffuse and change phongs parameters like IOR and exponent.

I would like to see the ability to add in a background image and then be able to adjust it with simple widgets..

Not possible without restarting the render..

Changing the background image would be like changing an env map in mid render I dont see that being possible.

True, but I can still see it saving people lots of time.. No need to do an alpha render first and then do some complex layer masking thing in photoshop. Having to restart the render really isn't much of a reason for not adding something. You will avoid having to go back to your exporter and fool with the settings before relaunching the render.

Which s not that much of a problem if changed on the fly


You just would have to think outside the box, as it's quite doable

Somehow I would question if that space outside the box also means onside of physical accuracy. Just because another render engine claims to be "physically based" and revs up their engine with all sorts of "features" does not mean they represent a physically valid approach.

Unless of course hey back that claim up with a white paper including research data.

neo. One reason why colors look different in an exporter preview than in an actual render is because the color of the rendered object are a direct result of it's physical properties. If you change the refractive, reflective etc. properties you'll change the color to some degree as well. If you follow Etienne's (galinette) posts closely you should certainly come to the conclusion that our physical world is quite a bit more complex than simply picking a color!

If you have your scene on memory and change values on the fly don't having to reload the scene from scratch is a major bonus, there's no black voodoo involved, and there's nothing biased or unbiased regarding this approach...

How do you figure that?! You think it's easier for me to reboot my 8 hour rendering to change or tweak a background image with a client breathing down my neck instead of rendering the scene, rendering an alpha map (which takes minutes) and compositing it in Photoshop in 7 different ways?

"Complex layer masking thing" isn't so complex at all, I use it every day, all the time, for pretty much every layer in an image.

Well, I stand corrected.