color of treeGrid column line

How do I change the color of the treeGrid column line ?

You can change column color whith setColumnColor(clr) method where clr - comma delimited colors list:
You can change background-color of the necessary row via XML or with API:

setRowColor(row_id, color) - sets background color of row (via bgcolor attribute):

setRowTextStyle(row_id, styleString) - sets style to row:
grid.setRowTextStyle(“r123”, “background-color: red; font-family: arial;”);