Further to a similar script for OmniGraffle:

Attaching a script like this to a keyboard shortcut, using something like Keyboard Maestro or Fastscripts, enables one to quickly switch the text color of selected rows. (This version affects only the topic text).

-- TOGGLE/CYCLE TEXT OF SELECTED ROWS BETWEEN BLACK, GRAY AND RED

property plstBlack : {0, 0, 0}
property plstGray : {32767, 32767, 32767}
property plstRed : {65534, 0, 0}

tell application id "com.omnigroup.OmniOutlinerPro3"
	try
		tell front document
			set idTopic to id of topic column
			set refRows to a reference to (selected rows where topic is not "")
		end tell
	on error
		return
	end try
	if (count of refRows) < 1 then return
	
	-- GET THE NEXT COLOR IN THE SEQUENCE
	tell (text of cell id idTopic of first item of refRows)
		set lstColor to its color
		if lstColor = plstGray then
			set lstNewColor to plstRed
		else if lstColor = plstBlack then
			set lstNewColor to plstGray
		else
			set lstNewColor to plstBlack
		end if
	end tell
	
	-- AND APPLY THE NEW COLOR TO ALL SELECTED ROWS
	set color of text of cell id idTopic of refRows to lstNewColor
end tell

Amended above - thanks to WhPalmer4 for diplomatically pointing out that it needs to be selected rows where topic is not "" - trying to toggle the color of a textless row would trip an error ...