NorthStar Echo Sounder 491 Operation Manual page 37

Hue and white level window
4. Press RETURN to go back to the ECHO screen.
You can replace any single color in the color bar with white.
Setting the white
On the HUE AND WHITE LEVEL window, press the CURSOR
level
PAD up or down to select that color.
To remove the white level and return to using all the colors on
the color bar, press the CURSOR PAD down until the white
level moves off the color bar.
491 Echo Sounder Operations Manual Page 33