Problem:
If there are changes, which require shoveling around lots of code, you may want to compare the moved code with the old code.
As an example look at https://gerrit-review.googlesource.com/#/c/65034/3
where code is moved from one method into a class. So there are lots of large chunks of code which are moved and due to the moving they are not diffed in the 5 color way, but show up as deleted and new code.
Possible solution:
You can select text the same way you do when commenting on a change. Instead of pressing 'c' for comment, you will press 'd' for diff and then you'll be asked to select another appropriate text section (which is most likely found on the other side of the screen). These 2 selections are then directly compared.
It should be using the 5 color scheme, maybe it could be in a popup or overlay?