I'm not seeing this problem either. What platform - hardware and operating system are you using?

Those labels are the "crosshair labels" that appear usually above the surfaces in suma. You can switch the default coloring from a white font with black shadows to a black font with white shadows with the F6 key to switch the background color. You can also change the font with this suma environment variable in your .sumarc file.

// 059-SUMA_CrossHairLabelFont:
//     Font for cross hair label in SUMA viewer
//      Choose one of: f8 f9 tr10 tr24 he10 he12 he18
//      
//     default:   SUMA_CrossHairLabelFont = f9
   SUMA_CrossHairLabelFont = he18

If you are missing the font indirectly named by these abbreviations, then I think you should see an error in the terminal. Trying another font might work then. You may need to reinstall these X11 fonts. That's OS dependent, so you might need to do something like:

sudo apt-get install -y  xfonts-base xfonts-100

or this larger list of fonts:

sudo apt-get install xfonts-base
sudo apt-get install xfonts-misc
sudo apt-get install xfonts-100dpi
sudo apt-get install xfonts-biznet-100dpi
sudo apt-get install xfonts-biznet-75dpi
sudo apt-get install xfonts-75dpi
sudo apt-get install xorg-x11-fonts-misc

And then restart your computer.
https://www.opengl.org/resources/libraries/glut/spec3/node76.html