Q:

How to have color in your blend file preview

So I am suffering from a TREMENDOUS amount of Blender files!  I currently have about 1000 on my machine and organizing these in certain folders helps a little but it does not really solve the issue.  I kept thinking I really need to get color in these grey previews and then one day I spotted this ... 

One file in hundreds just randomly has a little bit of color ... I have since tried everything to recreate this result somehow but have failed and I was just curious if anyone knew how to make these file previews colorful and easier on the eyes. 
Thank you !! 

A:

The File Browser preview only shows solid view colors. 
You have three options here:

Adjust the solid view color for each material (in Material tab)
Set solid view color for each object (in Object tab)
Use random color for each object

You can switch between different modes using the Viewport Shading menu.