If Line numbers is checked (enabled) in the View tab, this will display a column of row numbers on the left hand side of the edit window for each file. The font used is automatically selected based on the current font and is slightly smaller than the selected font for display.
Unchecking this option will visually hide the line numbers for all open files, although they are still maintained internally. To completely disable the tracking of line numbers, check the Disable line numbers option in Settings » Editor display » Miscellaneous.
The color of the line numbers can be configured in the Manage themes dialog and clicking on the Editor tab and then changing the color assigned to "Ruler, Line Number Column."
See also:
UltraEdit Wiki