Jonathan,
The grid lines are drawn with the color specified for the axis itself.
To make them with light grey, use color 16 ,17 or 18 for example.
Assume you want to draw an histogram hist and set the grid color:
TCanvas *c1 = new TCanvas("c1");
c1->SetGrid();
hist->Draw();
hist->GetXaxis()->SetAxisColor(17);
hist->GetYaxis()->SetAxisColor(17);
c1->RedrawAxis();
The last statement may be necessary to redraw the axis lines in
solid mode because the current grid algorithm may draw the grid lines
on top of the axis lines when they coincide.
Rene Brun