I cannot speak of TESB or ROTJ as to Lowry's involvement, but neither movie has a decaying negative. The boxed set of James bond films I have lists Lowry for the restoration on all of them, but only a few have any signs of a bad color balance.

Essentially what happens is that color negatives use different chemicals that are sensitive to the different wavelengths of light and that produce the appropriate color on the negative. These chemicals don't age at the same rate causing one color to fade faster than the rest. As a result, ANH looked green when they used the original color timing. Provided the color in question hasn't faded too much, the negative can be scannned and the colors adjusted during or after the scanning process (from my experience, you get better result when you adjust it during the scan). If the color has faded too much, the only option is to use a color separation or a print to retrieve that color. ANH is not that faded yet. Lucasfilm sent it to Lowry for the basic restoration. They did not get the colors aligned properly. Lucasfilm then went on to color correct the film, and TESB and ROTJ, and you can see the issues that they introduced in those as well, but the specific issues ANH has stem from the faded negative and Lowry's restoration. For for ANH, Lucasfilm's color correction is half the problem and Lowry's film restoration is the other. A few tweaks can fix Lucasfilm's bad judgement, but it is harder to dig out the fix to Lawry's bad work. I think I may have it, but I have just rendered my first draft color correction and haven't had a chance to look at it yet.