If you have multiple projects open in DVT and you are opening files from two different projects, it can get confusing which file belongs to which project, and perhaps you will mistakenly edit the wrong file.

To avoid this and change the file color of each project, you go to Window -> Preferences -> DVT and enable the option Color editor icons and view labels per project, and click Apply.

After that when you open files from two projects it should look like this:

The projects will also be different colors in Project Explorer:

You can also change the color by right clicking the project folder in Project Explorer then go to Project -> Change color.