Online colour grading LUTs So, I was researching some stuff about colour transforms, and found some companies who sell colour grading LUTs.
They did include with the free samples a set of transforms from a bunch of recording formats to rec709, so that the grading LUT will be applied to a rec709 input. 

I wasn't considering buying, but I was wondering how useful these would be. Given the expected pixel output 'x' depends on the transform performed on a given pixel input 'y'. Can we genuinely expect 'x' out for any 'y' put in after the conversion to rec709? 

If I have recorded for example, Alexa Log-C or RED Log, can I reasonably expect that my Alexa Log-C or RED Log pixel is the same as someone else's Alexa Log-C or RED Log pixel, to convert to rec709 and from there to an orange(or teal) 'film look' pixel?
You absolutely nailed it; they are a load of codswallop and rubbish.

The bottom line is that all colour transforms are based on a chain of assumptions. The moment a single assumption is wrong, or a single transform is out of order, the whole stack becomes invalid.

TL;DR is you can trust the transforms if you know they are the correct ones and they are / were applied properly. However, I would be at least as skeptical with purchased LUTs as one should be with HDRIs; prepare to be underwhelmed at best, and outright scammed at worst.

Fantastic question and welcome to the forum.

;)