You change the layer colors in 2 arrays:

#  set light_colors {
#   "#ffffff" "#0044ee" "#aaaaaa" "#222222" "#229900"
#   "#bb2200" "#00ccee" "#ff0000" "#888800" "#00aaaa"
#   "#880088" "#00ff00" "#0000cc" "#666600" "#557755"
#   "#aa2222" "#7ccc40" "#00ffcc" "#ce0097" "#d2d46b"
#   "#ef6158" "#fdb200" }

#  set dark_colors {
#   "#000000" "#00ccee" "#3f3f3f" "#cccccc" "#88dd00"
#   "#bb2200" "#00ccee" "#ff0000" "#ffff00" "#ffffff"
#   "#ff00ff" "#00ff00" "#0000cc" "#aaaa00" "#aaccaa"
#   "#ff7777" "#bfff81" "#00ffcc" "#ce0097" "#d2d46b"
#   "#ef6158" "#fdb200" }

The grid color is the third number. Oh, and you won't want the leading hash mark -- I just copied this straight out of the system xschemrc. To test things out before you commit it to your xschemrc, though, you can select the grid layer from the "Layers" menu, then go through "View" > "Change Current Layer color". Colors are selected with your run-of-the-mill hex RGB values.