In general, he based his color timing off of a fade free 1977 I.B. Technicolor Print, though he likely used a good amount of his own judgment throughout. There's a lot of crazy that was done to the colors during the shameful restoration done for the DVDs (with Blu-ray using the same dated master), so, many times even if most of a scene is made to match the reference colors, there could be subtle stuff that's still oddly color shifted he has to catch (I still take issue with what I swear is a green tint in the background of part of the droid cleaning / Leia projection scene).

For the Sandcrawler scene in question, I believe it is a particularly special case, as I think it is one he had to recreate from multiple sources... Correct me if I'm wrong, probably a custom matte from a film cell scan and an upscaled Sandcrawler imposed from the GOUT (2006 DVD). He probably tried to stay close to the I.B. reference on this scene, but the limited quality of the sources can drastically affect how it looks against the rest of the film. It likely took a lot of work to get it as close as he managed. I can't attest to how it originally was intended to look, but the more I look at the shadows, I do think the green/yellow tint there could stand to be diminished.