Is there a way to change the default color of comments that span across multiple lines?

Yes, in order to change the default color of comments that span across multiple lines in source code follow steps below:

1. From C/C++ perspective toolbar, select Preferences.

   The Preferences window appears.

2. Select C/C++ > Editor > Syntax Coloring.

   The Syntax Coloring page appears in the right panel (refer to the image listed below).

3. Select Comments > Multi-line comment.
   Figure 1. Preferences Dialog Box - Syntax Coloring
   
   
   
   
4. Select Color.

   The color palette window appears.

5. From Color Palette, select a color of your choice.
6. Click OK.
7. Click Apply.
8. Click OK.
   Note: If you want to change the color of a comment that is across a single line, then follow the steps given above, except selecting Single-line comment instead of Multi-line comment.

The color of the comments in the source code changes, as specified.