I have a suggestion that would make navigation much faster in some cases and I think it's probably not that big a deal to implement. At least I hope.
When you click on a part in a set's inventory, you are taken to a part page where you can see how many you have of that part by colour/partlist. But then, if you want to go to one of these specific colour/partlist page to edit the quantity, there is no direct way. You can link to the partlist and from there filter and search for the part but you already had found the part and the partlist so it seems like a link is missing somewhere.
Could the numbers in the colour/partlist grid be links to this page? Admittedly, I'm less sure of the implications for the grid in the part summary popup window but I imagine that would probably be relatively straightforward for the full page version.
I can see many cases where that would be useful but here's the one I most often face. I'm putting together a used set and some parts are missing. If I don't have the part in my loose parts, I mark it as lost. That goes well. But if I find that I have some loose ones of this part, I will instead reduce my number of loose parts since some are now assigned to this set to replace the missing ones. That's when I find myself doing the running around I described above and could go much faster with these links.