I am currently working on a desktop application handling photos. One of the screens we have is a view where you can see a list of albums sorted by date to the left, and when clicking on an album, a list of thumbnails for this album shown up to the right, see the sketch below. If the number of thumbnails exceeds one row, it will create a new one (this does not happen very often as the number of thumbnails are generally small). 

![Basic concept](https://i.stack.imgur.com/DqPVn.jpg)

Now, our problem is this: As it is possible and desirable for our users to view thumbnails for more than one date at the same time, this makes it very hard to see which date on the left corresponds to a particular set of thumbnails to the right once you get 3+ open and you need to start scrolling. Even if you can fit 5 dates in the view to the right, it can be confusing to see which date corresponds to which set of thumbnails. 

What we have done so far is to:

1. Show a small icon in the list to the left indicating if the thumbnails are showing or not.
2. Show a title bar above each set of thumbnails with date and a custom tag.
3. Always open them in chronological order.

Still, this does not feel like enough. Do you have any suggestions of how we can better show the correspondence between dates to the left and the thumbnails they refer to in the view to the right?

Restriction:
Coding the different thumbnails sections with different color would not be an option because of our graphical profile and the fact that it will sometimes be used on gray-scale monitors. However, something that would be in different shades of the same color would work.